Hydrocarbons — Short Questions

Organic compounds are the compounds which contain covalently bonded carbon as an essential component. They are found in living things i.e., plants and animals. For example: carbohydrates and proteins.

Inorganic compounds are made up of almost all elements of periodic table except those compounds of carbon and hydrogen (hydrocarbons). They are found in non-living things i.e., rocks and minerals. For example: NaCl and CaCO₃.

Organic compounds are found in large numbers due to unique properties of carbon.
i. The self-linking property of carbon called catenation is responsible for the existence of large number of organic compounds ranging from short to very long chains and large rings.
ii. Due to small size of carbon that can form strong bonds with the small atoms like hydrogen, oxygen and nitrogen etc.

When natural gas is oxidized under controlled conditions the products depends upon the degree & oxidation and reaction condition such as temperature, pressure and catalyst etc. Excess supply of oxygen gives CO₂ + H₂O & with heat.
CH₄ + 2O₂ →CO₂ + 2H₂O + Heat

Fractional distillation of petroleum gives naphtha which consists of a mixture of liquid hydrocarbons. It is then heated at around 500°C in the presence of catalyst called zeolite to give Hydrocarbons which have five to ten carbon atoms.

The molecular formula of iso-butane is C₄H₁₀.
The structural formula of iso-butane is:
H H H
| | |
H−C−C−C−H
| | |
H H
|
H−C−H
|
H

The condensed formula of iso-butane is:
H₃C−CH−CH₃
|
CH₃

Organic compounds are useful for us in many ways such as:
i. They are major constituents of our diet as carbohydrates, proteins and fats etc.
ii. The clothes we wear are made up of natural or synthetic fibre which are organic compounds.
iii. Fossil fuels used to run transport industry contain hydrocarbons which are organic compounds.

Five naturally occurring organic compounds are:
1. Methane 2. Glucose 3. Citric acid
4. Starch 5. Nucleic acids (DNA & RNA)

The longest continuous chain contain four carbon singly bonded to each other. So, its root name will be But− and suffix will be −ane. 2−methyl butane /iso-pentane.

When we move from lower to higher member of alkanes their melting and boiling points increase because of increase in molecular size there is increase in the attractive forces among molecules.

Structural formula of a compound represents the exact arrangement of the different atoms of various elements present in a molecule of a substance, e.g., Structural formula of butane is:
H H H H
| | | |
H− C − C − C − C − H
| | | |
H H H H
(n-butane)

(i) (CH₃)₂CH−CH−CH₂−CH₃ with CH₃ attached at positions 2 and 3
The longest continuous chain has five carbons singly bonded and two methyl groups are attached at carbon number two and three. So, name: will be 2,3-dimethyl pentane.

(ii) (CH₃)₂CH−CH₂−CH₃
The longest continuous chain has four carbons singly bonded and one methyl groups are attached at carbon number two. So, name: will be 2-methyl butane.

C₂H₅Br H−C−C−Br with H atoms above and below
H H

CH₃−NH₂ H−C−N with H atoms bonded
H H
H

CH₃CH₂OH H−C−C−O−H with H atoms bonded
H H

Methane has only one methyl (CH₃−) group. Ethane has two methyl (CH₃−) groups & propane has two methyl (CH₃−) and one methylene (−CH₂−) group.

During the reduction of alkyl halides using Zn/HCl, Hydrogen is oxidized.
R−X + 2[H] −Zn/HCl→ R−H + H−X

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O + Heat
C to C, C to H and O to O bonds are broken and C to O and H to O bonds are formed.

When methane reacts with chlorine gas, a series of reactions takes place as.
CH₄ + Cl₂ −IN→ CH₃Cl + HCl
CH₃Cl + Cl₂ −IN→ CH₂Cl₂ + HCl
CH₂Cl₂ + Cl₂ −IN→ CHCl₃ + HCl
CHCl₃ + Cl₂ −IN→ CCl₄ + HCl
CH₂Cl₂, CHCl₃ and CCl₄ are formed along with CH₃Cl

Hydrocarbons are organic compounds of carbon and hydrogen elements. They are alkanes, alkenes, alkynes and aromatics. e.g: CH₄,C₂H₆ etc.

Hydrocarbons are those compounds which are made up of only carbon and hydrogen elements. Hydrocarbons are regarded as the parent organic compounds since other organic compounds are considered to be derived from them by replacing of one or more hydrogen atoms by other atoms or group of atoms.

CNG stands for compressed natural gas and LPG stands for liquified petroleum gas.

Hydrocarbons are classified into structurally as:
• Alkanes
• Alkenes
• Alkynes
• Aromatics

Alkanes are saturated hydrocarbons in which all the carbon atoms are connected by only single covalent bond.
Examples:
Methane: H−C−H with H above and below
H

Ethane H−C−C−H with H atoms bonded
H H

This distinguishing feature of alkanes make them distinct from other compounds is their lack of reactivity towards usual chemical reagents.

The simplest hydrocarbons are alkanes, while simplest alkane is methane having formula CH₄.
Uses: It is mainly used as a fuel but it is also used to make hydrogen gas, carbon black, carbon disulphide, chloroform, hydrocyanic acid, etc.

H H H H H H H
| | | | | | |
H−C−C−C−C−H H−C−C−C−H
| | | | | |
H H H H H−C−H
| |
H H

Isopentane Isobutane

Alkanes are saturated hydrocarbons. In these compounds all the bonds of carbon atoms are single that means valencies of carbon atoms are fully satisfied (saturated). Therefore they are least reactive. That is the reason alkanes are called paraffins.

IUPAC stands for International Union of Pure and Applied Chemistry.

(i) Root: It tells us the number of carbon atoms in the longest continuous chain present in the molecule.
(ii) Suffix: It is added after the root and tells us about the class of organic compounds.
(iii) Prefix: It is indicated before the root and tells us about the group or groups attached to the longest chain.

Dot and Cross structures of Alkanes

Methane H−C−H with dot and cross notation showing electron pairs
H
x
H×C×H
x
H

Ethane H−C−C−H with dot and cross notation
H H
H H H H
H−C−C−H and H×C×C×H
H H H H

Propane H−C−C−C−H with dot and cross notation
H H H H H H
H−C−C−C−H and H×C×C×C×H
H H H H H H

Reduction means addition of nascent hydrogen. In fact, it is a replacement of a halogen atom with a hydrogen atom. This reaction takes place in the presence of Zn metal and HCl / e.g.
CH₄−Cl + 2[H] −Zn/HCl→ CH₄−H + H−Cl

What do you know about hydrogenation of alkenes?
Ans. Hydrogenation means addition of molecular hydrogen to an unsaturated hydrocarbon in presence of a catalyst (Ni, Pt) to form saturated compound e.g.
CH₂= CH₂ + H₂ −Ni/200°C→ CH₃— CH₃
(Ethene) (Ethane)

Cracking of petroleum helps balance the availability of petroleum fractions with the demand for them. When cracking transforms bigger hydrocarbons break up into smaller hydrocarbons. In this way, the fuel supply is increased, that helps to balance demand of supply.

Margarine is produced by adding molecular hydrogen to vegetable oil at 200°C in the presence of catalyst (Ni).
vegetable oil + H₂ −Ni/200°C→ Margarine
Greater the amount of hydrogen is added, the more solid the margarine becomes.

Complete combustion takes place in excess of air or oxygen to produce CO₂, H₂O & large amount of heat.
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

Alkanes burn in the presence of excess of air or oxygen to produce a lot of heat, carbon dioxide and water. This reaction takes place in automobile combustion engines, domestic heaters and cooking appliances. It is highly exothermic reaction and because of it alkanes are used as fuel. e.g.
CH₄ + 2O₂ → CO₂ + 2H₂O + heat

Those reactions in which products are formed by the addition of a substance like H₂, Cl₂, etc to an unsaturated hydrocarbon.
Example: Addition of hydrogen to an unsaturated hydrocarbon in the presence of catalyst (Ni, Pt) at 250 – 300°C.
CH₂= CH₂ + H₂ −Ni/200°C→ CH₃— CH₃
(Ethene) (Ethane)

The unreactivity of alkanes may be explained on the basis of the strength of single bond which is difficult to break. A significant amount of energy is required to break these bond for a reaction to occur

Natural gas primarily contain methane which is very flammable. (gives combustion reaction readily when ignited) that is why mixture of natural gas and air explode and a large amount of heat is released along with CO₂ and water vapours:
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat

Organic compounds are molecular in nature and have weak intermolecular forces, having covalent bond so generally organic compounds have low melting and boiling points. On the other hand inorganic compounds mostly contain ionic bonds and have strong attractive forces. So inorganic compounds have comparatively high melting and boiling points.

Reactions of alkanes with chlorine follow free radical mechanism. Sunlight fall on Cl₂ molecule and generates a chlorine free radical which attack on alkane molecule to form alkyl halide. That is why sunlight is quite necessary for reaction of chlorine with alkane molecule.

Straight chain structured compounds have strong intermolecular forces than branched chain compounds and hence higher boiling point. n-butane has straight chain of four carbon atoms. While iso-butane has straight chain three carbon atoms with one carbon as a branch. So, n-butane has higher boiling point than iso-butane.

Organic compounds are mostly non-polar in nature. According to general principle of solubility "like dissolve like" non-polar compounds are soluble in non-polar solvents while polar compounds are soluble in polar solvent. As water is a polar solvent, so generally organic compounds are not soluble in water.