Hydrocarbons — Long Questions

Uses of organic compounds No doubt, thousands of organic compounds are synthesized naturally by animals and plants. But millions of organic compounds are being prepared in the laboratories by the 'chemists. Because these compounds are part of everything from food we eat to the various items we use in daily life to fulfil our needs.

i. Uses as Food: The food we eat daily such as milk, eggs, meat, vegetables, etc., contain carbohydrates, proteins, fats, vitamins, etc., are all organic stuff.

ii. Uses as Clothing: All types of clothing (we wear, we use as bed sheets etc.) are made up of natural fibers (cotton, silk and wool, etc.) and synthetic fibers (nylon, dacron and acrylic, etc.) all these are organic compounds.

iii. Uses as Houses: Wood is cellulose (naturally synthesized organic compound). It is used for making houses and furniture of all kinds.

iv. Uses as Fuel: The fuels we use for automobiles and domestic purposes are coal, petroleum and natural gas. These are called fossil fuels. All of these are organic compounds.

v. Uses as medicines: A large number of organic compounds (naturally synthesized by plants) are used as medicines by us. Most of the life saving medicines and drugs such as antibiotics (inhibit or kill microorganisms which cause infectious diseases) are synthesized in laboratories.

vi. Uses as Raw Material: Organic compounds are used to prepare a variety of materials, such as rubber, paper, ink, drugs, dyes, paints, varnishes, pesticides, etc.

All the organic compounds are known to contain carbon as an essential element. This fact has led us to define organic chemistry as the chemistry of carbon compounds. Apart from carbon, most of the organic compounds contain hydrogen and oxygen as well.

Organic compounds are famous for their large number and diverse behavior. Several million organic compounds are known to exist naturally or have been synthesized in the laboratory. Organic molecules are usually large and more complex in nature. They include life molecules like proteins, enzymes carbon hydrates, lipids, vitamins and nucleic acid, pharmaceuticals and synthetic fibres, etc.

The number of compounds formed by the element carbon is far more than the total number of compounds formed by all the rest of the elements put together. This is due to come unique properties of carbon.

The element carbon is present at the center of the periodic table and it is energetically not possible for it to gain or lose electrons to form ionic bond. Therefore, it forms four covalent bonds. Because of their small size, these covalent bonds are short and strong enabling carbon to give strong and stable bond with itself and with hydrogen, oxygen and nitrogen. The self-liking property of carbon is called catenation and due to this it forms long, straight and branched chains and rings.

All these facts suggest that due to diversity of carbon compounds, carbon as an element is very important and a separate branch of chemistry shared be based upon it.

"An atom or group of atoms or presence of double or triple bond which determines the characteristic properties of an organic compound is known as the functional group".

Example -OH (Hydroxyl group) is the functional group of alcohols which give characteristic properties of alcohols.

Reasons

i. Reactivity
C-C Single Bond: A single bond is relatively stable and non-reactive under normal conditions. It doesn't introduce significant chemical properties to a molecule.

C=C Double Bond The double bond is weaker and more reactive than a single bond. This reactivity makes the double bond a site for various chemical reactions, such as addition reactions (e.g., hydrogenation, halogenation).

ii. Influence on Molecular Properties
C-C Single Bond: It doesn't significantly affect the physical or chemical properties of a molecule.

C=C Double Bond It affects the molecule's shape and its electronic properties, which influence how the molecule interacts in chemical reactions.

iii. Role in Functional Group Definition
A functional group is defined as an atom or group of atoms within a molecule that determines its chemical reactivity. The double bond fits this criteria, because it:
1. Dictates how a molecule reacts.
2. Is a key site for chemical reactions.

In contrast, a single bond only connects atoms without imparting giving chemical properties.

iv. Functional Group Classification
The carbon-carbon double bond is the defining feature of alkenes, a class of organic compounds with distinct reactivity and properties. Single bonds, being inert, do not define a specific class of compounds based on reactivity.

As a result of the great complexity and large number of organic compounds, it is not possible to name each and every compound individually. The International Union of Pure & Applied Chemistry has devised a systematic way of naming organic compounds called IUPAC nomenclature.

According to IUPAC system of nomenclature, the entire name of an organic compound has three parts:

i. Root: It tells us the number of carbon atoms in the longest continuous chain present in the molecule. The roots up to ten carbon atoms are shown in table.

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.

To explain the above system, let us name the following compound.
CH3—CH — CH2 — CH3
|
CH3

a) Identify the longest continuous chain present in the compound.
b) Identify the class of organic compounds.
c) Identify the substituent or substituents if present.

This organic compound contains four carbon atoms in the longest continuous chain and it belongs to the family of organic compounds called Alkane. The root is therefore But- and the suffix-ane added to this. The organic compound will thus be given the name Butane.

CH3 — CH2 — CH — CH3
|
CH3

The name of the only branch methyl-will be added to this name as prefix.
So the name will become:

1CH3—2CH—3CH2—4CH3
|
CH3
Branch

To specify where the branch occurs, the longest continuous chain is numbered starting from the end closest to the branch. This number is then attached to this prefix. The name of the above compound will then be:

1CH3—2CH—3CH2—4CH3
|
CH3

2-Methylbutane or iso-Pentane

If compound has no branches, its name will contain only root and suffix.

Combustion Alkanes burn in oxygen or air to form CO2 and H2O with the evolution of large amount of heat.

CH4(g)+ 2O2(g) ——→ CO2(g)+ 2H2O(g)+ heat
2CH3– CH3(g) + 7O2(g) ——→ 4CO2(g) + 6H2O(g) + heat

Types of Combustion

i. Complete Combustion: Occurs in the presence of sufficient oxygen, producing carbon dioxide and water as products. This releases a large amount of energy.
CH4 + 2O2 ——→ CO2+2H2O+heat

ii. Incomplete Combustion. Occurs when oxygen supply is limited, leading to the formation of carbon monoxide (CO) and carbon black along with water. This releases less energy and can be harmful.
3CH4 + 4O2 ——→ 2CO+6H2O+ C

Importance of Combustion of Alkanes

The combustion of alkanes has several practical uses and benefits in our daily lives.

i. Energy Production
• Alkanes, especially methane, propane, and butane, are used as fuels in homes and industries.
• e.g. Methane (natural gas) is used in cooking.
• Propane is used in gas cylinders in stoves.
• Large-scale combustion of alkanes in power plants generates electricity.

ii. Transportation
Gasoline and diesel, which are mixtures of alkanes, are used fuels in cars, trucks and aeroplanes. Their combustion provides the energy required to power engines.

iii. Industrial applications
Alkanes are used as fuels in various industries, such as steel manufacturing and chemical production. For instance:
Combustion of alkanes provides the heat needed for melting metals.
It drives machinery in factories.

Natural gas is widely used in kitchens for cooking because it is a clean and efficient source of energy. However, if the gas valve is left open without being ignited, it can spread throughout the kitchen. This situation is highly dangerous and can lead to an explosion.

Why Does an Explosion Occur?

Natural gas, primarily composed of methane (CH4), is highly flammable. This means it can easily catch fire when it comes into contact with a spark or flame. Methane reacts rapidly with oxygen in the air, releasing a large amount of energy in the form of heat and light. A small spark, flame, or even heat from an electrical device (like a switch, bulb, or appliance) can ignite the gas-air mixture, causing a sudden and violent combustion an explosion.

Consequences of a Gas Explosion

The force of the explosion can destroy walls, windows, and appliances in the kitchen.
The explosion can start a fire, spreading further damage.
The sudden explosion can cause severe injuries or even fatalities.

How Can You Avoid a Gas Explosion?

To prevent such accidents, follow these safety measures:

i. Always ensure the gas valve is turned off when not in use. Double-check the valve after cooking to confirm it is closed.

ii. Keep your kitchen well-ventilated. If you suspect a gas leak, open all doors and windows to allow the gas to disperse quickly. Leave the area and call gas technician to address the issue.

iii. Inspect your gas appliances, pipes, and valves regularly for leaks or damage. Replace worn-out or damaged components immediately.

iv. Install a gas leak detector in your kitchen. These devices can sense the presence of gas and alert you before it becomes dangerous.

v. Teach everyone in the household about gas safety, including how to identify a gas smell and what to do in an emergency.

Medicinal Benefits of the Neem Tree

Neem is a well-known tree that is commonly grown throughout many parts of the world, including our country. It is often referred to as the "Village Pharmacy" because almost every part of this tree e.g. leaves, bark, seeds, and oil has medicinal properties. Neem is widely used in traditional medicine and plays an important role in promoting health and treating various health problems.

Medicinal Benefits of Neem

i. Neem is highly effective in killing bacteria and viruses. Its extracts are used to treat skin infections, wounds, and acne. Neem oil is applied to cuts and scratches to prevent infection and promote healing.

ii. Neem paste or neem water helps reduce pimples and other skin conditions. It is a common ingredient in soaps, creams, and face washes because of its cleansing and purifying effects.

iii. Neem twigs are traditionally used as toothbrushes. Chewing on these twigs helps clean teeth, prevent cavities, and maintain oral hygiene. Neem also fights gum infections and strengthens teeth.

iv. Consuming neem leaves or neem-based medicines helps purify the blood. This improves overall health and reduces the risk of infections.

v. Neem has compounds that help lower blood sugar levels. Drinking neem tea or consuming neem extracts is beneficial for managing diabetes.

vi. Regular use of neem strengthens the immune system, helping the body fight off diseases.

vii. Neem oil is a natural insect repellent. It is used to protect crops from pests and to keep mosquitoes and other insects away from humans.

viii. Neem helps treat stomach problems like ulcers, constipation and intestinal worms. It also promotes a healthy digestive system.

ix. Neem reduces swelling and pain in condition like joint problems.

x. Neem oil or neem shampoo helps control dandruff, lice and other scalp infections. It promotes healthy hair growth and keeps the scalp nourished.

Few Popular Medicines

Several medicines use use organic compounds, as they contain carbon-based structures essential for their biological activity.

i. Aspirin (Acetylsalicylic Acid)
It is a pain reliever, anti-inflammatory and fever reducer. Aspirin inhibits the enzyme which is involved in the production of chemicals that cause pain, inflammation and fever.

ii. Paracetamol (acetaminophen)
It is an analgesic (pain reliever) and antipyretic (fever reducer). Paracetamol acts on the brain to regulate body temperature and reduce fever. It also blocks certain chemical pathways in the brain to reduce the sensation of pain.

iii. Penicillin
It is an antibiotic. It is effective against a range of bacterial infections, including pneumonia and throat problem.

iv. Morphine
It is an opioid painkiller. It is used to manage severe pain, such as in cancer patients or after surgery. However, it can be addictive and requires careful use.

v. Insulin
It is used for hormonal treatment for diabetes. Insulin helps regulate blood sugar levels by facilitating the uptake of glucose into cells for energy or storage.

Generally any member of the alkane series can be prepared by the following methods.

(i) Cracking of higher Hydrocarbons
Cracking is a process in which hydrocarbons with higher molecular masses are broken up into smaller hydrocarbons which are more useful. This is done by heating the hydrocarbons at high temperature in the presence of a catalyst 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.

Naphtha Heat/Zeolite→ Alkanes and alkenes containing 5 to 10 carbon atoms

Mixture of hydrocarbons

(ii) Reduction of Alkenes and Alkynes:
Alkanes can be prepared by reducing alkenes and alkynes with hydrogen gas in the presence of nickel metal as a catalyst. Methane cannot be prepared by this method. The reaction is also called hydrogenation of alkenes and alkynes and is an example of addition reaction. An addition reaction occurs when hydrogen (H2) is added to an unsaturated compound.

CH2= CH2 + H2 Ni/200°C→ CH3—— CH3
Ethene Ethane

CH ≡ CH + 2H2 Ni/200°C→ CH3—— CH3
Ethyne Ethane

(iii) Reduction of Alkyl Halides:
Alkyl halides (R-X) can be reduced to alkanes with hydrogen generated by reaction of zinc metal with hydrochloric acid.

R - X + 2[ H ] Zn/HCl→ R - H + H - X

CH3 - Cl + 2[ H ] Zn/HCl→ CH - H + H - Cl

Substitution reactions Alkanes give substitution reactions. The reactions which involve the replacement of hydrogen of alkanes by an atom or a group of atoms like halogen are called substitution reactions.

Halogenations of alkane Alkanes react with halogens especially chlorine to give alkyl halides. Since these substitution reactions are carried out in the presence of sunlight, are called chemical substitution reactions.

CH4 + Cl2 hv→ CH3 — Cl + H — Cl
Methane Chloromethane

The reaction may proceed ahead and all the hydrogen atoms attached with carbon of the methane are successively replaced by chlorine atoms.

CH3— Cl + Cl2 hv→ ' CH2Cl2 + HCl
Dichloromethane
CH2Cl2 + Cl2 hv→ CHCl3 + HCl
Dichloromethane or chloroform
CHCl3 + Cl2 hv→ CCl4 + HCl
Tetra chloromethane or carbon tetrachloride