Chapter 17: Crude Oil and Hydrocarbons

Organic Chemistry 1 (Crude Oil and Hydrocarbons)

Fractional distillation of crude oil

Crude oil is a mixture of a variety of liquid and gaseous hydrocarbons with different boiling points. By using fractional distillation, we can separate crude oil into its different useful fractions.

Each fraction contains a mixture of hydrocarbons which has a different boiling point range. Crude oil is heated in a furnace and vapourised. Hot vapour flows up the fractionating column. As it cools down, each fraction condenses when the temperature of the column is lower than its boiling point.

– Lighter fractions with lower boiling point ranges will distil at the top of the column

– Heavier fractions with higher boiling point ranges will distil at the bottom of the column

 

Trend of increasing carbon atoms in homologous series or crude oil fractions

Melting and boiling point: Increasing number of carbon atoms → more electrons → intermolecular forces become stronger → more energy required to overcome → higher melting / boiling point

Density: Increasing number of carbon atoms → higher density

Viscosity: Increasing number of carbon atoms → more electrons → intermolecular forces  become stronger → more difficult for substance to flow → higher viscosity

Flammability: Increasing number of carbon atoms → more bonds are required to break → lower flammability

Question 2 (Admiralty Sec)

Answer

The fractions are arranged in increasing boiling points from W to Z. This means W is distilled at the top of the fractionating column and has the smallest M_r while Z is distilled at the bottom and has the highest M_r.

B is not the answer as Y has more carbon atoms and electrons than X, resulting in stronger intermolecular forces. This makes Y more difficult to flow, thus Y should be more viscous than X.

C is not the answer as Z has more carbon atoms and thus should have a higher molecular mass than X.

D is not the answer as Z has more carbon atoms and thus should have a longer chain length than X.

A is the answer as Y has more carbon atoms and bonds that are required to break than W; thus Y should be less flammable than W.

Introduction (Homologous Series)

Organic compounds are compounds that contains carbon and hydrogen. Sometimes they can also contain oxygen, nitrogen or halogens. Organic compounds are grouped into homologous series.

Homologous series:

A family of organic compounds which have

1. the same general formula with

2. each member differing from the next by a -CH₂ group.

3. the same functional group and hence,

4. have similar chemical properties.

Isomers: Compounds with the same molecular formulae but different structural formulae.

Question 1 (Admiralty Sec)

The structural formula of compound Z is shown.

Answer

Compound Z has 5 carbon, 12 hydrogen and 1 oxygen atoms. An isomer of Z would have the same molecular formula of C₅H₁₂O but different structural formula.

Both A and B is not the answer as they have a different molecular formula of C₅ H₁₀O.

D is not the answer as it has a different molecular formula of C₄H₁₀O.

C is the answer as it has a same molecular formula of C₅H₁₂O with a different structural formula (the way the atoms are arranged are different).

 

Naming of organic compounds

Organic compounds are named from the number of carbon atoms which determines the prefix and the functional group which determines the suffix.

Alkanes

They are hydrocarbons (only containing carbon and hydrogen) with the general formula C_nH_2n+2.

Characteristics

•  low melting / boiling points
•  insoluble in water
•  soluble in organic solvents
•  saturated (C-C)

Chemical properties of alkanes

1. Combustion

Examples:

•  Complete combustion: CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (𝑙)
• Incomplete combustion: 3CH₄ (g) + 5O₂ (g) → CO₂ (g) + 2CO (g) + 6H₂O (𝑙) 

        or

• 2CH

₄

• (g) 3O

₂

• (g) → CO

₂

• (g) + C (s) + 4H

₂

O (𝑙)

Reactants: Oxygen

Conditions: 

Alkanes tend to undergo incomplete combustion as the number of carbon atoms increase.

2. Substitution

Examples:

•  CH₄ + C𝑙₂ → CH₃C𝑙 + HC𝑙
•  CH₄ + Br₂ → CH₃Br + HBr

Reactants: C𝑙₂ (g) or Br₂ (𝑙)

Conditions: UV    light

Alkanes can go through multiple substitutions to form CH₂C𝑙₂, CHC𝑙₃, CC𝑙₄

 

Alkenes

They are hydrocarbons (only containing carbon and hydrogen) with the general formula C_nH_2n and contain the functional group C=C.

Characteristics

•  low melting / boiling point
•  insoluble in water
•  soluble in organic solvents
•  unsaturated (C=C)

Formation of alkenes

Alkenes are manufactured by cracking, which involves breaking down long-chain hydrocarbons into short-chain hydrocarbons and/or hydrogen.

Examples:

• C₁₈H₃₈ → C₈H₁₆ + C₁₀H₂₀ + H₂
• C₁₂H₂₆ → C₄H₈ + C₈H₁₆ + H₂

Reactants: –

Conditions:  A𝑙₂O₃ or SiO₂ powder as catalyst, high temperature (500°C)

 

 

Chemical properties of alkenes

1. Combustion

Examples:

• Complete combustion: C₂H₄ (g) + 3O₂ (g) → 2CO₂ (g) + 2H₂O (𝑙)

Reactants: Oxygen

Conditions: 

A higher percentage of carbon in alkenes causes it to combust with a sootier flame as compared to alkanes

2. Addition

 

 

 

3. Polymerisation

 

Question 3 (Singapore Sports Sch)

Q is a compound that:

• can be formed, as the only other product, when the alkane C₈H₁₈ is cracked to produce butane,
• decolourises bromine water,
• has a branched chain structure.

Answer

From the 1^st statement, C₈H₁₈ follows the general formula of alkanes. C₈H₁₈ is cracked to form butane (C₄H₁₀) and another compound (Q = C₄H₈, butene).

From the 2^nd statement, Q is unsaturated and has a C=C bond which decolorises bromine water. Hence, options B and C are eliminated.

From the 3^rd statement, Q is a branched compound.

D is the only answer as it fits all 3 criteria.