Organic Chemistry

11.1 Formulae, functional groups and terminology

Organic chemistry is the study of compounds of carbon. Carbon atoms form four covalent bonds and join together in chains.

Types of formula

Other structural formulae: ethene $\ce{CH2=CH2}$ and methyl ethanoate $\ce{CH3COOCH3}$. In a displayed formula each line is a single covalent bond, and a double line is a double bond.

Homologous series

A functional group is an atom or group of atoms that determines the chemical properties of a homologous series.

$\ce{C_nH_{2n+2}}$ with $n = 8$ gives $\ce{C8H18}$ (since $2n + 2 = 18$).

Saturated and unsaturated

• A saturated compound has molecules in which all the carbon–carbon bonds are single bonds (alkanes).
• An unsaturated compound has molecules in which one or more carbon–carbon bonds are not single bonds, for example $\ce{C=C}$ (alkenes).

Structural isomers

11.2 Naming organic compounds

The stem of the name shows the number of carbon atoms in an unbranched chain, and the ending shows the homologous series.

The number shows the position of the double bond or the $\ce{-OH}$ group. Number from the end that gives the lower number. For example, propan-1-ol is $\ce{CH3CH2CH2OH}$ and propan-2-ol is $\ce{CH3CH(OH)CH3}$.

Esters. An ester is named from the alcohol and the acid: the alcohol gives the first word (alkyl) and the acid gives the second ($\text{-oate}$). Ethanol and ethanoic acid give ethyl ethanoate, $\ce{CH3COOCH2CH3}$, and methanol and propanoic acid give methyl propanoate, $\ce{CH3CH2COOCH3}$.

It has four carbons (but-) and a $\ce{C=C}$ double bond starting at carbon $1$, so it is but-1-ene, an alkene.

11.3 Fuels

The fossil fuels are coal, natural gas and petroleum (crude oil). Methane is the main constituent of natural gas.

A hydrocarbon is a compound that contains hydrogen and carbon only. Petroleum is a mixture of hydrocarbons.

Fractional distillation of petroleum

Petroleum is separated into useful fractions by fractional distillation in a fractionating column.

1. The petroleum is heated and the vapour enters near the bottom of the column.
2. The column is hot at the bottom and cooler at the top.
3. The vapours rise, and each fraction condenses where the temperature is just below its boiling point, and is drawn off. Substances with large molecules condense low in the column and those with small molecules condense high in the column.

A fraction is a mixture of hydrocarbons with similar boiling points (similar chain length).

Trends from the bottom to the top of the column

Uses of the fractions

11.4 Alkanes

Alkanes are saturated hydrocarbons. The bonding is single covalent. Their general formula is $\ce{C_nH_{2n+2}}$.

Alkanes are generally unreactive, because the $\ce{C-C}$ and $\ce{C-H}$ bonds are strong. They react in two ways: combustion and substitution by chlorine.

Combustion. In a plentiful supply of air, alkanes burn completely to carbon dioxide and water:

$\ce{CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)}$

In a limited supply of air, incomplete combustion gives carbon monoxide or carbon (soot) as well.

Substitution. In a substitution reaction, one atom or group of atoms is replaced by another atom or group of atoms. Alkanes react with chlorine in the presence of ultraviolet light:

$\ce{CH4 + Cl2 -> CH3Cl + HCl}$

$\ce{C2H6 + Cl2 -> C2H5Cl + HCl}$

The reaction is photochemical: the UV light provides the activation energy, $E_\text{a}$. Only one hydrogen atom is replaced when we say monosubstitution, so the products are chloromethane (or chloroethane) and hydrogen chloride.

11.5 Alkenes

Alkenes are unsaturated hydrocarbons with a $\ce{C=C}$ double bond, which makes them more reactive than alkanes. Their general formula is $\ce{C_nH_{2n}}$.

Cracking

$\ce{C10H22 -> C8H18 + C2H4}$

Why crack? There is more demand for shorter-chain hydrocarbons (such as petrol) than the amount obtained from fractional distillation, and the alkenes produced are the starting materials for polymers and other chemicals.

Testing for unsaturation

The test for an alkene uses aqueous bromine (bromine water).

Addition reactions

In an addition reaction the alkene adds another substance across the double bond, and only one product is formed. The $\ce{C=C}$ becomes a single bond.

$\ce{CH3CH=CH2 + H2 -> CH3CH2CH3}$

The product is propane.

11.6 Alcohols

Ethanol, $\ce{C2H5OH}$, can be made in two ways.

Combustion. Alcohols burn in air to carbon dioxide and water:

$\ce{C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(l)}$

Uses of ethanol: as a solvent (in perfumes and inks) and as a fuel (alone or mixed with petrol).

11.7 Carboxylic acids

Carboxylic acids contain the $\ce{-COOH}$ group. They are weak acids and react like other acids, with the salt named after the acid, "$\text{-oate}$":

Making ethanoic acid

Ethanol is oxidised to ethanoic acid:

• by heating with acidified aqueous potassium manganate(VII), which changes from purple to colourless, or
• by bacterial oxidation in air, which is how vinegar is made from wine or beer.

$\ce{CH3CH2OH + 2[O] -> CH3COOH + H2O}$

Esters

A carboxylic acid reacts with an alcohol, with an acid catalyst (concentrated sulfuric acid), to make an ester and water. The reaction is reversible.

$\ce{CH3COOH + CH3CH2OH <=> CH3COOCH2CH3 + H2O}$

The ester ethyl ethanoate has a sweet, fruity smell. Esters are used in perfumes and food flavourings.

11.8 Polymers

Addition polymers

In addition polymerisation, the monomers are unsaturated (alkenes). The double bonds open up and the monomers join in a long chain, and no other product is made.

$n\,\ce{CH2=CH2} \to \left[\ce{-CH2-CH2-}\right]_n$

To find the repeat unit of an addition polymer, take the alkene's two carbon atoms with the groups on them, open the double bond into a single bond, and show the bonds extending out of the brackets. To find the monomer from a polymer, reverse this.

Condensation polymers

In condensation polymerisation, two different monomers, each with two functional groups, join, and a small molecule (water) is lost each time a link forms.

PET can be converted back into its monomers and re-polymerised, so it is recycled.

Proteins

Proteins are natural polyamides. They are made from amino acid monomers, which have the general structure $\ce{H2N-CHR-COOH}$, where R is a side-chain that is different in different amino acids. The amino acids join by amide links ($\ce{-CONH-}$), with the loss of water.

Plastics and the environment

Most plastics are not biodegradable, so they cause problems when disposed of:

• in landfill sites, they last for hundreds of years and take up space;
• they accumulate in the oceans, harming and killing marine life, and breaking into tiny pieces (microplastics);
• when burned, they produce toxic gases, for example carbon monoxide, and hydrogen chloride from PVC.

The properties of plastics (strong, light, not rotting) make them useful, but their disposal is a challenge. Recycling helps, as does using fewer plastics.