Unit 11: Hydrocarbons — Long Questions

Hydrocarbons
Definition: Hydrocarbons are organic compounds made up of only carbon (C) and hydrogen (H) atoms.

Natural Occurrence Found abundantly in petroleum and natural gas.

Uses of Hydrocarbons

• Used as domestic and industrial fuels.
• Have many synthetic applications.
• Serve as raw materials for: Medicines, plastics, perfumes, polymers, unsaturated hydrocarbons especially alkenes are starting materials in industrial synthesis.
• Used in the production of various commercial products.

Classification of Hydrocarbons Hydrocarbons are broadly divided into:
(i) Aliphatic Hydrocarbons
(ii) Aromatic Hydrocarbons

Explanation

• Hydrocarbons contain only carbon and hydrogen.
• They are major components of fossil fuels.
• Play a vital role in the chemical industry.
• Serve as precursors to numerous synthetic materials.

ALIPHATIC AND AROMATIC HYDROCARBONS

(i) Aliphatic Hydrocarbons
Definition: Aliphatic hydrocarbons are organic compounds that do not contain aromatic rings.

Examples

• Open-chain (acyclic) or closed-chain (cyclic)
• Saturated (alkanes) or unsaturated (alkenes and alkynes)
• If they contain only carbon and hydrogen atoms, they are called aliphatic hydrocarbons.

Types
(a) Acyclic Aliphatic Hydrocarbons: They are open chain hydrocarbons.

• Alkanes: Saturated hydrocarbons (methane, ethane) CH₄, CH₃ – CH₃ etc.
• Alkenes: Unsaturated hydrocarbons with at least one double bond (ethene) CH₂ = CH₂, (propene) CH₂ = CH– CH₃ etc.
• Alkynes: Unsaturated hydrocarbons with at least one triple bond (ethyne) CH ≡ CH, (propyne) CH ≡ C – CH₃ etc.

(b) Cyclic aliphatic hydrocarbons (cycloalkanes): cyclopropane, cyclobutane, cyclopentane and cyclohexane.

(ii) Aromatic Hydrocarbons
Definition: Aromatic hydrocarbons are a special class of cyclic compounds characterized by a high carbon-to-hydrogen ratio and delocalized electrons (resonance) based on benzene (C₆H₆) and its derivatives.

Examples Benzene, methylbenzene (toluene), ethylbenzene, phenol and naphthalene etc.

Alkanes or Paraffins
Definition: Alkanes are saturated hydrocarbons containing only single bonds between carbon and hydrogen atoms.

• General formula: CₙH₂ₙ₊₂
• Simplest alkane: Methane (CH₄)
• They are present organic compounds.
• The homologous series of alkanes with condensed formula and molecular formula are shown in table 11.1.

Table 11.1 Condensed structural formulae and molecular formulae of alkanes shows methane through decane with their molecular formulas and condensed structures.

ALKYL GROUPS
Definition: When one hydrogen atom is removed from an alkane, the remaining part is called an alkyl group. Named by replacing the "-ane" suffix of alkanes with "-yl".

Table 11.2 Alkyl groups their names and abbreviations
Methane (CH₄) → Methyl (CH₃–) (Me–)
Ethane (CH₃–CH₃) → Ethyl (CH₃–CH₂–) (Et–)

For alkanes with more than two atoms, more than one alkyl group can be derived.
Example: Propyl can give n–propyl and iso–propyl.

Table 11.3 Alkyl groups, Straight-chain and branched and their names shows propane giving n-Propyl and Iso-Propyl groups, and butane giving n-Butyl, sec-Butyl, iso-Butyl, and tert-Butyl groups.

IUPAC Naming Rules
To systematically name branched alkanes, follow these eight IUPAC rules:

Rule 1 Identify the longest continuous chain
Locate the longest continuous chain of carbon atoms; this chain determines the parent name for the alkane. We designate the following compound as a hexane because the longest continuous chain contains six carbon atoms, that is a bent chain.

Example That the following alkane is designated as a heptane because the longest chain contains seven carbon atoms.

• If the longest chain has 7 carbon atoms → heptanes

Rule 2 Number the chain from the end nearest a substituent
Number the longest chain beginning from the end nearest to the substituent. Applying this rule, we number the two alkanes shown below in the following way.

Rule 3 Indicate the position of the substituents
Use the numbers obtained by applying rule 2 to designate the location of the substituent group. The parent name is placed last, and the substituent group, preceded by the number designating its location on the chain, is placed first.
Example: 3-Methylheptane

Rule 4 Alphabetical order for multiple substituents
When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain.
Example: We designate the following compound as 4-Ethyl-2-methylhexane.

The substituent groups should be listed alphabetically (i.e. ethyl before methyl). While deciding on alphabetical order ignore multiplying prefixes such as "di" and "tri"

Rule 5 Two substituents on the same carbon
When two substituents are present on the same carbon atoms, use that number twice.
Example: 3-Ethyl-3-methylhexane

Rule 6 Identical substituents
When two or more substituents are identical, indicate this by the prefixes di, tri, tetra, and so on. Then make certain that each and every substituent has a number
Examples: 2,3-Dimethylbutane, 2,3,4-Trimethylpentane, 2,2,3,3-Tetramethylpentane.

Applying these six rules allows us to name most of the alkanes that we shall encounter. Two other rules, however, may be required occasionally.

Rule 7 Choosing the Parent Chain
When two chains of equal length compete for selection as the parent chain, choose the chain with the greater number of substituents.
Example: 2,3,5-Trimethyl-4-iso-propylheptane

Rule 8 First point of difference rule
When branching first occurs at an equal distance from either end of the longest chain, choose the name that give the lower number at the first point of difference.
Example: 2,3,5- Trimethylhexane (not – 2,4,5 – Trimethylhexane)

Nomenclature of cycloalkanes
Cycloalkanes are named by attaching the prefix 'cyclo' to the names of the alkanes possessing the same number of carbon atoms.
Examples: For three carbons shape = cyclopropane for four carbons shape = cyclobutane etc.

Structure of Alkanes

• Each carbon atom in an alkane is sp³ hybridized.
• It forms four sigma (σ) bonds arranged tetrahedrally and bond angle 109.5°. C-C bond length is 1.54.

Bonding

• Alkanes are often shown using ball-and-stick models to illustrate 3D geometry.
• Carbon atoms in alkanes are surrounded by four pairs of bonding electrons which repel equally to each other to form an angle of 109.5.

The shapes of some straight chain alkanes are shown in table 11.4 with methane, ethane, propane, and butane structures.

Structure of cycloalkanes Cycloalkanes are cyclic meaning that the carbon atoms of the molecules are arranged in the form of a ring.
Definition: Cycloalkanes are saturated cyclic hydrocarbons.

• All C-C bonds are single bonds forming a closed ring.
• Each carbon is sp³ hybridized and forms 4 sigma bonds.
• The structural shapes of cyclopropane, cyclobutane and cyclopentane are shown in table 11.5.

Physical properties of alkanes
(i) Physical state and appearance

• C₁ to C₄ (methane to butane) are colourless, odourless, gases at room temperature.
• C₅ to C₁₇ (pentane to heptadecane) are colourless, odourless liquids. The higher members from C₁₈ to onwards are above waxy solids.

(ii) Solubility and polarity

• Alkanes are non-polar or very weakly polar.
• Insoluble in polar solvents like water.
• Soluble in non-polar solvents such as hexane, benzene and diethyl ether etc.

(iii) Variation of physical constants with size

• The physical constants like boiling point, melting point, and density increase with the increase in number of carbon atoms.
• Solubility decreases as molecular size increases.

(iv) Effect of branching on boiling point

• Branched-chain alkanes have lower boiling points than their straight-chain isomers. Example: n-Butane has low boiling point (−0.5 °C) than 2-Methylpropane (isobutane) boiling point (−11.7 °C). Reason: Branched molecules have less surface area, leading to weaker intermolecular forces (Van der Waals interactions).

Reaction Mechanism
Definition: A reaction mechanism is the step-by-step sequence of elementary steps or reactions by which an overall chemical change occurs.

Every organic reaction involves bond breaking and bond formation.
Covalent bonds in organic molecules can break in two main ways:

(i) Homolytic fission
In homolytic fission, the covalent bond breaks evenly to produce free radicals and each atom retains one electron from the shared pair.

• A free radical is a species with an unpaired electron and is highly reactive.
• Reason: Free radicals are highly reactive due to the tendency of unpaired electrons to pair up.

Example
Cl—Cl → Cl• + Cl• (Free radicals)

(ii) Heterolytic fission
In heterolytic fission, the covalent bond breaks unevenly, and one atom takes both electrons from the bond.

• Produces oppositely charged ions (a cation and an anion).
• Shown in mechanisms using full curly arrows to indicate electron pair movement.

Example
H–C–Cl → H–C⁺ + :Cl⁻

UNREACTIVE NATURE OF ALKANES TOWARDS POLAR REAGENT

The alkanes or paraffins The term "paraffins" comes from Latin: Parum means little, Affinis means affinity.
This name reflects their low chemical reactivity, especially towards polar reagents.

Inertness of Alkanes
Under ordinary conditions, alkanes are inert or unreactive towards acids, alkalis, oxidizing agents and reducing agents.

Reasons for Unreactivity
(i) Non-Polarity of Bonds

• Alkanes are composed of C-C and C-H bonds.
• Electronegativity of carbon is 2.5 and hydrogen is 2.1.
• The small difference in electronegativity makes these bonds non-polar.
• Polar reagents do not interact easily with alkanes.

(ii) Stability of sigma (σ) bonds

• The bonds in alkanes are sigma (σ) bonds.
• In σ-bond, electrons are tightly held between two nuclei making it very stable bond.
• Breaking a σ-bond requires a large amount of energy, contributing to alkane inertness.

Reactions of alkanes
Under high temperature or suitable conditions, alkanes undergo two main types of reactions.
(i) Thermal and catalytic reactions: These reactions typically occur at high temperatures or in the presence of catalysts.
Examples: Combustion and cracking.
(ii) Free radical substitution reactions: These reactions involve the replacement of one atom or group of atoms by another proceed via a free radical mechanism.
Example: Halogenation of alkanes.

FREE RADICAL SUBSTITUTION REACTIONS

Free radical reactions
Halogenation: The substitution of a halogen atom with hydrogen atom of an alkane called halogenation.

• These reactions occur in the presence of light energy (UV light), forming highly reactive free radicals.
• Alkanes react with chlorine (Cl₂) or bromine (Br₂) in sunlight or UV light.
• Hydrogen atoms are successively replaced by halogens.
• The extent of halogenation depends on the amount of halogen used.

Reactivity of halogens
Order of reactivity: F₂ > Cl₂ > Br₂ > I₂

• Fluorine (F₂): Reacts violently to produces carbon, fluorinated alkanes, and hydrofluoric acid (HF).
• Chlorine (Cl₂) and Bromine (Br₂): Undergo substitution in presence of UV light.
• Iodine (I₂): Reaction is too slow and reversible.

Mechanism of halogenation (Three steps)

(i) Initiation Step

• It is the first step in the mechanism of free radical substitution of alkanes by halogen.
• Halogen molecule breaks into radicals using UV light: Cl—Cl → Cl• + Cl• (with UV light)

(ii) Propagation step

• It is the second step in free radical substitution.
• Radicals formed in initiation step attack the alkanes, generating more free radicals and continuing the chain. H–C–H + Cl• → H–C• + H–Cl H–C• + Cl—Cl → H–C–Cl + Cl•
• This step repeats to produce various chloroalkanes.

(iii) Termination Step: It is the final step in the free radical substitution in which two free radicals combine to form a product.
The termination may take place in different ways:

• The chloride and alkyl radicals may combine to form chloroalkanes.
• Two chloride may react to form chlorine molecule.
• Alkane with double number of carbons may formed by merging two alkyl radicals, such products formed in traces and it is rare in this mechanism.

(iii) Cl• + Cl• → Cl– Cl
By repetition of step two, a mixture of chloroalkanes (halogen substituted alkanes) are produced. The chlorine gas in the reaction mixture to start with, the greater proportions of chlorine atoms in the chloroalkanes molecules formed.

Final Products

• A mixture of halogenated alkanes.
• More chlorine is higher degree of substitution.
• Trichloromethane (Chloroform)
• Tetrachloromethane (Carbon tetrachloride)

Further substitution with Cl radical:
CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄
Chloromethane → Dichloromethane → Trichloromethane → Tetrachloromethane

Alkenes

• Alkenes are unsaturated hydrocarbons.
• They consist only of carbon (C) and hydrogen (H).
• Contain at least one carbon-to-carbon double bond (C=C).
• Alkenes have two fewer hydrogen atoms than the corresponding alkanes and their general formula is CₙH₂ₙ.

IUPAC rules for naming Alkenes
The IUPAC nomenclature for alkenes is largely similar to that of alkanes, with a few key modifications:

(i) Select the longest carbon chain containing the double bond
Select the longest continuous chain that contains the C = C as the parent chain. Change the ending of the name of the alkane of identical length from – ane to – ene.

Example
H₂C–CH₂–CH₃ with double bond between carbons
3 – n – Propyl – 2 – hexene

(ii) Number the carbon chain
Number the chain so as to include both carbon atoms of the double bond are included in the chain. Numbering begins from the end nearer to the double bond.

Example 5 – Methyl – 2 – hexene

(iii) Indicate the position of the double bond
Designate the location of the double bond by using the number of the first atom of the double bond as a prefix.
Example: 2-Butene (double bond starts at carbon 2)
H₂C = CH–CH₂–CH₃: 1–Pentene

(iv) Identify and number substituent groups
Indicate the locations of the substituent groups by the numbers of the carbon atoms to which they are attached.

Example 2-Methyl-2-butene and 2,5,5-Trimethyl-2-hexene.

(v) Naming alkenes with more than one double bonds

• "diene" for two double bonds (But-1,3-diene)
• "triene" for three double bonds (Hept-1,3,5-triene)
• Indicate the positions of all double bonds in the name.

Example Names

• CH=CH–CH=CH₂ (But-1,3-diene)
• CH₃–CH=CH–CH=CH–CH=CH–CH₂ (Hept-1,3,5-triene)

Preparation of Alkenes
Alkenes are commonly prepared through elimination reactions. The chemical reactions in which small atoms or groups (like H, OH, or X) are removed from adjacent carbon atoms of a saturated compound, resulting in the formation of a carbon-carbon double bond (C=C).

Elimination reactions

• Small molecules such as H₂O or HX (HCl, HBr) are eliminated.
• These reactions convert saturated compounds (alcohols, or alkyl halides etc) into unsaturated alkenes.

Methods of preparation
(i) Dehydration of Alcohols

• Ethanol is dehydrated using concentrated sulfuric acid (H₂SO₄) at 180°C.
• A water molecule is removed from ethanol to form ethene.

Reaction
CH₃CH₂OH → CH₂=CH₂ + H₂O
Ethanol heat Ethene water

Mechanism Ethanol reacts with H₂SO₄ to form ethyl hydrogen sulfate. Upon heating, this intermediate loses H₂SO₄, forming ethene.
CH₃CH₂OH+H₂SO₄ → CH₃–CH₂–OSO₃H+H₂O (180°C)
CH₃CH₂OSO₃H → H₂C=CH₂+H₂SO₄ (Heat)

(ii) Dehydrohalogenation of Alkyl Halides

• Ethene is formed by heating ethyl halide (like ethyl bromide) with alcoholic KOH.
• Elimination of hydrogen and halogen from adjacent carbon atoms occurs to form a double bond.

Reaction
H₂C—CH₂—Br + KOH(alcoholic) → H₂C=CH₂ + KBr+H₂O

PHYSICAL PROPERTIES OF ALKENES

Alkenes display certain distinct physical properties due to the presence of the carbon–carbon double bond (C = C) and the nature of their molecular structure.

(i) Physical state

• The first three alkenes like ethene, propene, and butane are gases at room temperature.
• Alkenes with 5 to 15 carbon atoms (C₅–C₁₅) are liquids. Higher alkenes (C₁₆ and above) are solids.

(ii) Solubility

• Insoluble in water (due to non-polar nature).
• Soluble in organic solvents like alcohol, ether, and benzene.

(iii) Odour and combustion

• Alkenes have a characteristic smell.
• They burn with a luminous (yellow, smoky) flame due to incomplete combustion and higher carbon content.

(iv) Polarity

• Unlike alkanes, alkenes exhibit weak polarity.
• This is due to the sp² hybridization of carbon atoms in the double bond, which slightly affects electron distribution.

Structure of Ethene (C₂H₄)

• The simplest alkene is ethane.
• Each carbon atom in ethene is sp² hybridized.
• Three equivalent sp² orbitals are formed by mixing one 2s orbital, two 2p orbitals which are present in the same plane.
• One unhybridized p-orbital remains on each carbon atom lies perpendicular (90°) to the plane of hybrid orbitals.

Geometry and Bonding

• In ethene each carbon used sp² orbitals to form two C–H bonds each.
• The remaining sp² orbitals forms C–C σ bonds by linear overlap of sp² orbitals.
• The unhybridized p-orbitals which are parallel to each other (one on each carbon) overlap sideways to form a π bond.
• A double bond (C=C) consists of one σ bond and one π bond.
• The two carbon atoms forming a double bond and four H – atoms attach directly to them lie in the same plane.

Molecular Shape

• The molecule has a trigonal planar geometry around each carbon.
• All atoms connected to the C=C bond.
• Bond angles are approximately 120°.

Illustrations
[Structure shows ethane with bonds and 120° angles]

Reactivity of the π bond
(i) Nature of the π bond

• Alkenes are more reactive than alkanes due to presence of the double bond.
• The π-electrons are located above and below the plane of the bonded atoms, not directly between the nuclei.
• The probability of finding electrons is away from the line joining the two nuclei.
• This makes the π bond weaker and more easily broken than a σ bond.

(ii) Electrophilic attack

• Because the π electrons are more exposed, they readily attacked by electrophilic (electron-seeking) reagents.
• This leads to electrophilic addition reactions, which are characteristic of alkenes.
• As a result, electrophiles (electron-seeking species) can easily attack the π bond.

Ans. Stability of carbocation
A carbocation is an alkyl group where a carbon atom carries a single positive charge. It is highly unstable and reactive, typically formed during electrophilic addition reactions.

Types of carbocation
(i) Methyl carbocation (CH₃⁺)
Positive charge on carbon bonded only to hydrogen atoms named as methyl carbocation (CH₃⁺).

(ii) Primary (1°) carbocation
Positive carbon is bonded to one other carbon atom (or alkyl group) named as primary (1°) carbocation.

(iii) Secondary (2°) carbocation
Positive carbon is bonded to two carbon atoms (alkyl groups) directly is named as secondary (2°) carbocation.

(iv) Tertiary (3°) carbocation
Positive carbon is bonded to three carbons (alkyl substituents) is named as tertiary (3°) carbocation.
More alkyl groups means greater stability.

Inductive effect
Definition: The polarization of a σ bonds due to electron-donating or withdrawing effect of adjacent groups or atoms is referred to as inductive effect.
The inductive effect of alkyl group play a vital role in carbocation stability.

Electron donating inductive effect (-I effect)
Definition: The electron donating groups, such as alkyl groups (1°, 2°, 3° carbocation), exhibit a positive inductive effect denoted as (+I).
• Alkyl groups exhibit +I or positive effect. Electron donating species pushing electron density toward the positively charged carbon.
• It reduces the intensity of the positive charge, making the carbocation more stable. The more alkyl groups attached to the carbocation, the more stable it becomes.
• The alkyl group attached to the positively charged carbon atoms are electron donating groups.
• Due to the positive charge on the carbon atom, carbocation is electrophile.
• The inductive effects is shown by the arrowheads on the bonds to show the alkyl groups pushing electron towards the positively charged carbon.
• Forming carbocation energetically more stable by making it less positively charged.
• By increase in number of alkyl groups the stability of carbocation increases due to +I effect.
• The presence of three electron donating alkyl groups make tertiary carbocation most stable energetically.

Order of stability

H—C⁺ < R→C⁺ < R→C⁺ < R→C⁺
| | | |
H H H R
↑

Type of Carbocation | Attached Groups | Inductive Effect | Stability
Methyl | No alkyl groups | No +I effect | Least stable
Primary (1°) | 1 alkyl group | Weak +I effect | Low
Secondary (2°) | 2 alkyl groups | Moderate +I effect | Medium
Tertiary (3°) | 3 alkyl groups | Strong +I effect | Most stable

Ans. Reactions of Alkenes
Alkenes are unsaturated hydrocarbons (contain at least one carbon–carbon double bond C=C).
• Due to the presence of this π-bond, alkenes are more reactive than alkanes.
• The π-electrons are loosely held and can be easily attacked on electrophiles.

Electrophilic addition reactions
Definition: Electrophilic addition reactions are those in which an electrophile (X⁺) is added across the double bond (C=C) of an alkene.
• The C=C double bond in alkenes consists of a strong sigma (σ) bond and a weaker pi (π) bond and more exposed.
• The π electrons are electron-rich, making the double bond a site of high electron density.
• This attracts electrophiles (electron-deficient species), initiating the reaction.

General mechanism of electrophilic addition

Step 1 Attack on electrophile
• The electrophile (X⁺) is attracted to the π-electrons of the double bond.
• The π-bond breaks, and X⁺ adds to one of the carbons, forming a carbocation intermediate.

Step 2 Attack by nucleophile
• The nucleophile (Y⁻) quickly attacks the carbocation, resulting in the final addition product and the completion of addition reaction.

• X⁺ is the electrophile (H⁺, Br⁺)
• Y is the nucleophile (Br⁻, Cl⁻, OH⁻)

Electrophile Electron-deficient species (X⁺) that initiates the reaction.
Nucleophile: Electron-rich species (Y⁻) that completes the reaction.
The intermediate is a carbocation.

Halogenation
Definition: Halogenation is the addition of halogen molecules (like Cl₂ or Br₂) across the double bond of an alkene, forming vicinal dihalides (halogens on adjacent carbon atoms).

Test for unsaturation
• Bromine water (reddish-brown in color) is used to test for the presence of a C=C double bond.
• When ethene is bubbled through bromine water in the presence of an inert solvent like carbon tetrachloride, CCl₄, the reddish-brown color disappears, indicating addition across the double bond and identifying unsaturation of an organic compound.

Reaction

CH₂ = CH₂ + Br₂ —CCl₄→ Br – CH₂ – CH₂ – Br
1,2-Dibromoethane

Mechanism of Halogenation The above reaction proceeds through a three step mechanism.

Step 1 Induced Dipole Formation
• Br₂ is a non-polar molecule, but as it approaches the electron-rich C=C double bond, the high electron density repels the electrons in Br–Br bond, creating a temporary dipole.
• One Br atom becomes partially positive (δ⁺) which is closes to the double bond acts as an electrophile.

Step 2 Electrophilic Attack
• The π electrons from the alkene attack the electrophilic Br⁺, leading to the formation of a highly reactive cyclic brominuim ion or a carbocation intermediate.

Step 3 Nucleophilic Attack
• The Br⁻ ion attacks the carbocation or brominuim ion formed in step 2.
• Final product is a vicinal dibromide, 1,2-dibromoethane.

Ans. Hydrohalogenation
Definition: Hydrohalogenation is the addition of a dry gaseous hydrogen halide (HX) at room temperature to an alkene, resulting in the formation of a halogenoalkane (alkyl halide).

Example
CH₂ = CH₂ + HBr —→ CH₃ – CH₂ – Br
Ethene Bromoethane

Properties of hydrogen halides (HX)
• HX are the simplest example of polar molecules with a permanent dipole.
• The H atom is electrophilic (δ⁺), and X⁻ (halide ion) is nucleophilic.
• All HX (HF, HCl, HBr, HI) react with alkenes, exactly the same way.
• Reaction rate increases from HF to HI (HI gives the fastest reaction).
• The actual product formed depends upon weather the alkenes is symmetrical or unsymmetrical.

(i) Addition to symmetrical alkenes: Only one product is feasible, when alkene is symmetrical.

Mechanism steps
The mechanism of addition of HBr to ethene is:

Step 1 Nucleophilic π electrons of the C=C bond attack the hydrogen of HBr. Br⁻ ion is formed by HBr.
A carbocation intermediate is formed by adding hydrogen to one of the double bonded carbon of ethene.

Step 2 Br⁻ (nucleophile) attacks the carbocation to gives the final product.

(ii) Addition to symmetrical alkenes
Two products are possible, but one is major, due to Markovnikov's rule, when alkenes are unsymmetrical.

Markovnikov's rule
"When a polar reagent adds to an unsymmetrical alkene, the negative part of the reagent (X⁻) attaches to the double bonded carbon which bears lesser number of hydrogen atoms."

Mechanism for propene and HBr
Propene reacts with HBr in two ways. The mechanism for this reaction involves the following steps.

Step 1
• π bond attacks H⁺ (Hydrogen ions) adds to the carbon with more H atoms.
• Hydrogen bromide forming bromide (Br⁻) ion by breaking heterolytically.
• A more stable secondary carbocation is formed, by adding hydrogen to double bonded carbon with more number of hydrogens.

Step 2
• Br⁻ attacks the carbocation to give 2-Bromopropane (major product).
• Minor path, less stable primary carbocation forms 1-Bromopropane.

Hydrogenation
Definition: Addition of molecular hydrogen (H₂) across the double bond of an alkene in the presence of a metal catalyst (Ni, Pt, Pd) at 250–300°C, forming a saturated alkane is called hydrogenation.

General Reaction H₂C=CH₂ + H₂ —Ni, 250-300°C→ H₃C–CH₃

Industrial use This reaction is used to convert vegetable oil into margarine (Banaspati ghee)
(Hydrogenation turns unsaturated fats into saturated fats.)

oil + H₂ —Ni→ Margarine (Banaspati Ghee)

Ans. (i) Hydration
Definition: Addition of steam (H₂O) to alkenes to form alcohols, using concentrated sulfuric acid (H₂SO₄) as a catalyst.
• Similar to hydrohalogenation, carbocation intermediate is formed, OH⁻ (from water) attacks the carbocation.

Example
H₂C=CH₂ + H₂O —H₂SO₄→ C₂H₅OH
Ethanol (alcohol)

(ii) Halohydration
Definition: Addition of halogen (Cl₂ or Br₂) and water to an alkene to form a halohydrin (contains –OH and –X on adjacent carbons).

Example The reaction specifically produces bromohydrin and chlorohydrin for ethene, depending upon halogen used.
H₂C=CH₂ + Br₂ + H₂O —→ CH₂ – CH₂ + HBr
| |
Br OH

(iii) Epoxidation
Definition: The epoxidation of alkenes gives an oxygen containing three-membered cyclic ether, called an epoxide.

Explanation
• The reaction can be completed using various oxidizing agents in case of ethene (C₂H₄).
• Ethene can also be converted to ethylene oxide using molecular oxygen and metal catalysts.
• A transition metal catalyst facilitates the reaction, forming an epoxide directly from the alkene.
• The epoxides can be converted in to diols by acid hydrolysis.

H H H H OH
| | + | | H⁺/H₂O | |
C = C O₂ —Ag→ C — C ———→ H₂C — CH₂
| | | | | |
H H O OH OH

Epoxide Ethane-1,2-diol

Uses Intermediate in the production of glycols, plastics, detergents.

(iv) Ozonolysis
Definition: Oxidative cleavage of the C=C double bond using ozone (O₃) a reactive allotrope of oxygen to form ozonides, which break down into carbonyl compounds (aldehydes, ketones), then alcohols upon reduction.

Identification of alkenes It is done by breaking alkenes into smaller and more identifiable pieces, by using this reaction. The location of double bond on carbon change can be identified by this reaction.

Mechanism Alkene reaction with ozone to form ozonide which can be further reduced to carbonyl compounds, and ultimately to alcohols.

Example
C₂H₄+O₃ —→ C₂H₄O₃ —→ 2CH₂O
Zn/H₂O

O—O O O
/ Zn/H₂O || ||
H₃C—CH=CH₂ —O₃→ H₃C—HC CH₃ ———→ H₃C—C—H + H—C—H
/ Ethanal Methanal
O
1-Propene

reduction
↓
H₃C—CH₂—OH CH₃—OH
Ethanol Methanol

Ans. Polymerization
Definition: Polymerization is the formation of extremely long molecules (polymers) from small reactive molecules that join together (monomers).
• Addition polymerization: The basis of plastic industry is the addition polymerization reaction that are the most important reactions of alkenes.
• Addition polymerization is a chemical reaction where many monomer units, each containing a carbon-carbon double bond (C=C), join together to form a polymer a large molecule with repeating units, without producing any by-products.
• π-bond in each C=C bond breaks and then monomers link together to form a new C-C single bonds, just like other addition reactions of alkenes.
• Polymer: A polymer is a long change molecule that is made up of many repeated units.

Examples of Polymerization
(i) Ethene to Poly-ethene

Monomer n molecules of ethene undergo polymerization to form polyethylene polymer. This is used in plastic bags, containers, packaging.

(ii) Chloroethene into polyvinylchloride (PVC)

Chloroethene undergoes polymerization to form poly(chloroethene) (PVC). Used in pipes, electrical insulation, pens.

Deducing repeating units
Definition: A repeating unit is the smallest group of atoms that when connected one after the other makeup the polymer chain that repeats over and over to form the polymer.

Identification of repeating unit In polyethene and substituted polyalkenes (such as PVC) made of one type of monomers, the repeating unit is the same as the monomer except that the C=C double bond is changed to a C-C single bond.

H H H H H H H H
| | | | | | | |
n C = C ——→ —[ C — C ] ——→ —C — C — C — C—
| | | |ₙ | | | |
H H H H H H H H
Poly (ethene) Repeating unit
Ethene
Monomer Polymer

Ans. Isomerism
Definition: Two or more compounds having the same molecular formula but different structural formulas and properties are said to be isomers and the phenomenon is called isomerism.

Explanation Isomerism is the important property of organic compounds. The structural formulas show the arrangement of atoms and bonds in it.

Example of Alkanes
The simplest hydrocarbon to have structural isomers in butane (C₄H₁₀). Butane (C₄H₁₀) show two isomers which are:
• n-Butane CH₃–CH₂–CH₂–CH₃
• Isobutane (2-Methylpropane) CH₃–CH(CH₃)–CH₃.
• By the study the structural formula of butane and other higher hydrocarbons of alkanes, it is possible to arrange atoms present in the molecule in more than one way to satisfy all the valencies.
• It is possible to have two or more different arrangements for the same molecular formula.

Example of Chlorobutane Isomers
• 1-Chlorobutane: CH₃–CH₂–CH₂–CH₂–Cℓ
• 2-Chlorobutane: CH₃–CH(Cℓ)–CH₂–CH₃
• Both have same molecular formula C₄H₉Cℓ but different structures and properties, verified by experimental evidence.
• Isomerism is not only possible but common if the compound contain more than three carbon atoms.
• With increase in number of carbon atoms in hydrocarbons, the number of possible isomers increase very rapidly.
• The five carbon compound pentane, has three isomers.

Types of Isomerism

A. Structural Isomerism
Definition: The structural isomerism arises due to the difference in the arrangement of atoms within the molecule.
The structural isomerism can be explained in five different ways:

(i) Chain isomerism
This type of isomerism arises due to the difference in the nature of the carbon chain.

Example The pentane (C₅H₁₂) and butane (C₄H₁₀), have the following possible arrangements:
CH₃ – CH₂ – CH₂ – CH₃ CH₃ – CH – CH₃
|
CH₃
n – Butane 2 – Methylpropane
(Isobutane)

CH₃ – CH₂ – CH₂ – CH₂ – CH₃ CH₃ – CH – CH₃ CH₃
| |
CH₃ – CH₂ – CH – CH₃ CH₃ – C – CH₃
| |
n – Pentane 2 – Methylpropane CH₃
(Isopentane) 2,2 – Dimethylpropane
(Neopentane)

(ii) Position isomerism
Definition: Functional group remains the same, but its position changes on the carbon chain. So, this type of isomerism is called position isomerism.
The arrangement of carbon atoms remains the same.

Example chloropropane and butene (C₄H₈) can have two positional isomers.
CH₃ – CH₂ – CH₂ – Cl CH₃ – CH – CH₃
|
Cl
1 – Chloropropane 2 – Chloropropane

CH₃ – CH₂ – CH = CH₂ CH₃ – CH = CH – CH₃

But – 1 – ene But – 2 – ene

(iii) Functional group isomerism
Definition: The type of isomerism arises due to the same molecular formula but different functional groups are present is called functional group isomerism.

Example There are two compounds having the same molecular formula C₂H₆O, but different functional groups like ether and alcohols are present.
Carboxylic acids and esters are also the examples of this category.

H₃C – O – CH₃ H₃C – CH₂ – OH
Methoxy methane Ethanol

(iv) Metamerism
Definition: This type of isomerism arises due to the unequal distribution of carbon atoms on either side of the functional group.

Example Such compounds belong to the same homologous series, like diethyl ether and methoxy propane belongs the ether series.

H₃C – H₂C – O – CH₂ – CH₃ H₃C – O – CH₂ – CH₂ – CH₃
Ethoxy ethane Methoxy propane

(v) Tautomerism
Definition: This type of isomerism arises due to shifting of proton from one atom to another within the same molecule.

Example This is commonly seen in amino acids. They exist as zwitter ions which are formed when a proton is shifted from the carboxylic acid end of amino acid molecule to the amine group. Zwitter ion is also known as internal salt.

COO⁻ COO⁻
| ⇌ |
R — C — NH₂ R — C — NH₃⁺
| |
H H
Amino acid Zwitterion

B. Stereoisomerism
Definition: Stereoisomers have the same structural formula but differ in spatial arrangement of the identical groups in space.

Types There are two types of stereoisomerism.
(i) Geometrical or cis-trans Isomerism
(ii) Optical Isomerism.

Ans. Organic redox reactions
Definition: Oxidation-reduction reactions that involve the organic compounds are called organic redox reactions.
• The redox reactions are the transfer of electrons through addition or removal of atoms or bonds, specifically oxygen and hydrogen atoms.
• These reactions are characterized by the addition or removal of atoms or bonds.
• Oxygen is added or hydrogen is removed during organic oxidation.
• Hydrogen is added or oxygen is removed showing organic reduction.

Some example of oxidation and reduction reactions are given below.

1. Oxidation
(i) Combustion Reaction:
The reaction of excess of oxygen or air with organic compounds is usually called combustion or oxidation of organic compounds.
2C₂H₆ + 7O₂ —→ 4CO₂ + 6H₂O

Complete oxidation of ethane and ethene produces carbon dioxide and water.

(ii) Ozonolysis:
Definition: Oxidative cleavage of the alkenes (C=C double bond) using ozone (O₃) a reactive allotrope of oxygen.
Breaks double bonds, forming carbonyl compounds (aldehydes/ketones).

(iii) Epoxidation of alkenes
Definition: The epoxidation of alkenes gives an oxygen containing three-membered cyclic ether, called an epoxide.
• The epoxidation of alkene is also the example of oxidation reaction.
• Formation of epoxides from alkenes using oxidizing agents like O₂, peracids, or metal catalysts.

H H H H OH
| | + | | H⁺/H₂O | |
C = C O₂ —Ag→ C — C ———→ H₂C — CH₂
| | | | | |
H H O OH OH
Ethane–1,2–diol
Epoxide

(iv) Oxidation of alcohols
Definition: The primary alcohol (ethanol) oxidized to carboxylic acid using acidified K₂Cr₂O₇.

C₂H₅OH + 2[O] —K₂Cr₂O₇→ CH₃COOH + H₂O

2. Reduction
(i) Hydrogenation of alkenes
Definition: The addition of hydrogen to carbon-carbon double or triple bond in the presence of a catalyst and temperature is called hydrogenation.
• It converts alkenes into alkanes.
• Involves addition of hydrogen across a double bond.

H H H H
| | | |
C = C + H – H —Pt→ H – C – C – H
| | | |
H H H H

(ii) Reducing agents
There are large number of reducing agents are used in organic synthesis.
Some of them are lithium aluminum hydride (LiAlH₄), sodium borohydride (NaBH₄), tin and hydrochloric acid (Sn + HCℓ).

Common reductions
• Nitriles to amines
• Amides to amines
• Carboxylic acids to primary alcohols
• Nitrobenzene to phenylamine

Explanation Reduction of acetic acid (carboxylic acid) to ethanol (alcohol) using LiAlH₄. And aldehyde to alcohol using LiA/H₄ as well.

O OH
|| |
H₃C – C – H + 2[H] —LiAlH₄/Δ→ CH₃ – CH₂