4.1. Basic Concepts and Hydrocarbons: Module 4: Core Organic Chemistry: Chemistry OCR A A Level

Hydrocarbons

Organic chemistry mainly concerns the properties and reactions of hydrocarbons, compounds that contain only carbon and hydrogen atoms. Hydrocarbons form series of compounds with similar structures and formulas that can be represented in many different ways

Nomenclature

Nomenclature is the set of rules that outline how different organic compounds should be named and how their formulas are represented.

Naming Compounds

Compounds are named according to rules laid out by the International Union of Pure and Applied Chemistry (IUPAC). This ensures each compound is universally named the same - which helps to avoid potentially dangerous confusion.

Empirical Formula

The simplest whole number ratio of atoms of each element in a compound.

Molecular Formula

The true number of atoms of each element in a compound.

General Formula

The simplest algebraic formula of a member of a homologous series. All members of a homologous organic series follow the general formula.

Structural Formula

- The minimal detail that shows the arrangement of atoms in a molecule.

- The carboxyl group will be represented as COOH and the ester group as COO.

Displayed Formula

- The relative positioning of atoms and the bonds between them. Shows every atom and every bond in an organic compound.

Skeletal Formula

- The simplified organic formula of just a carbon skeleton and functional groups.

- Shows only the bonds in a compound and any non-carbon atoms.

- Vertices are carbon atoms.

- Hydrogen is assumed to be bonded to them unless stated otherwise.

Cyclic Compounds and Benzene

- These compounds are represented by the symbols below.

- Benzene can be represented in two equivalent ways.

Homologous Series

A homologous series is a series of organic compounds with the same functional group but with each successive member differing by CH2.

Organic Compounds

Organic compounds are often part of a homologous series, in which all members follow a general formula and react in a very similar way.

Alkyl Group

An alkyl group is a hydrocarbon chain with the general formula CH₂₊₁. Sometimes R may be used to represent alkyl groups, as well as other fragments of organic compounds not involved in reactions.

Aliphatic

An aliphatic compound is a compound containing carbon and hydrogen joined together in straight chains, branched chains or non-aromatic rings.

Alicyclic

An alicyclic compound is an aliphatic compound arranged in non-aromatic rings, with or without side chains.

Aromatic

An aromatic compound is a compound containing a fully conjugated ring. Benzene is aromatic.

Saturated

A compound is described as being saturated if it contains single carbon-carbon bonds only.

Unsaturated

A compound is described as being unsaturated if it contains multiple carbon–carbon bonds, including C=C, C≡C, and aromatic rings.

Isomerism

Isomers are molecules with the same molecular formula but a different arrangement of atoms within the molecule. This gives the isomers a different structural formula.

Chain Isomers

Chain isomers have the same molecular formula but a different arrangement of the carbon skeleton. The isomers’s carbon chain will differ - they can be straight chains or branched chains, however they will always have the same molecular formula.

Position Isomers

Position isomers have the functional group of the molecule in a different position on the carbon chain.

Functional Group Isomers

Functional group isomers have the same molecular formula, but the molecules have a different functional group.

Mechanisms

A ‘curly arrow’ is used to describe the movement of an electron pair during a reaction. It can show either heterolytic fission or the formation of a covalent bond.

Main types of reaction.

1. Addition

2. Substitution

3. Oxidation

4. Reduction

5. Polymerisation

6. Elimination

Addition

The reactants combine to form a single product.

Substitution

One functional group is replaced by a different functional group.

Oxidation

A species loses at least one electron, and is oxidised.

Reduction

A species gains at least one electron, and is reduced.

Polymerisation

A reaction in which many small molecules, known as monomers, join together to form a long, repeating molecule called a polymer.

Elimination

A small molecule is removed from a larger molecule.

Carbon monoxide

Carbon monoxide is a toxic, gaseous product that is especially dangerous to humans as it is odourless and colourless. Carbon monoxide is dangerous because it replaces oxygen in the blood, starving the brain and other organs of oxygen and causing people to suffocate.

Unsaturated hydrocarbons with at least one carbon-carbon double bond. They are part of a homologous series with the general formula CnH2n.

Alkenes and cycloalkenes.

Carbon-carbon Double Bond

The carbon-carbon double bond is an area of high electron density, making it susceptible to attack from electrophiles (species that are attracted to electron dense areas). It consists of a normal covalent σ bond and a π bond.

π-bond

The π-bond is formed by sideways overlap of adjacent p orbitals above and below the bonding C atoms.

σ-bond

The σ-bond is formed by overlap of orbitals directly between the bonding atoms.

C-H bond

The C-H bonds are in the same plane as the C-C σ bond. There is trigonal planar geometry around the carbon atom and a bond angle of 120°. This angle minimises electron-electron repulsion between bonding pairs.

Bromine Water

Bromine water is used to identify an alkene double bond and other unsaturated compounds. Alkenes cause bromine water to change colour from orange-brown to colourless.

Stereoisomers

Stereoisomers are compounds with the same structural formula but with a different arrangement of atoms in space.

E-Z isomerism

E-Z isomerism is a type of stereoisomerism, which occurs due to the limited rotation around a double carbon bond.

Limited Rotation

The limited rotation means that ‘high priority’ groups attached to the C=C can either be ‘together’ or ‘opposite’.

Haloalkanes

Haloalkanes are organic compounds with single carbon bonds only and halogen functional groups.

Br₂

Br₂ is present in bromine water in the test for unsaturation. This is an electrophilic addition reaction, forming a dihaloalkane.

Alcohols

Alcohols are organic compounds with a hydroxyl functional group, -OH. Alkenes undergo addition reactions with steam to form alcohols. This reaction requires an acid catalyst, such as phosphoric acid.

Electrophiles

Electrophiles are electron pair acceptors and are attracted to areas of high electron density.

Most common electrophile sources.

● HBr

● Br2

● H2SO4

Electrophilic Addition

Electrophilic addition is the reaction mechanism that shows how electrophiles attack the double bond in alkenes. When the double bond is broken, a carbocation intermediate forms. This is a carbon atom with only three bonds, so it has a positive charge.

Hydrogen Bromide

Hydrogen bromide is polar due to the difference in the electronegativities of hydrogen and bromine.

Addition Polymers

Addition polymers are produced from alkenes which are short chain monomers which join together to form long chain polymers.

Energy and resources.

The energy and resources used to make polymers are large.

Polymers

Polymers are made from alkenes which are obtained from crude oil, a non-renewable resource.

Polymers are unreactive hydrocarbon chains with multiple strong, non-polar covalent bonds. This makes them useful for manufacturing many everyday plastic products such as poly(ethene) shopping bags.