Quick Notes - AS Organic Chemistry

Organic Introduction

Organic Chemistry - Introduction

Empirical formula shows the simplest whole number ratio of elements within a molecule.
Molecular formula shows the actual number of atoms of each element in a molecule.
Structural formula shows how the atoms are arranged in a molecule.
Displayed formula shows a drawing of the arrangement of atoms in a molecule.
Skeletal formula shows the carbon backbone and functional groups within molecules, no carbon-hydrogen bonds are shown.

Carbon Chains

Prefixes are used when naming organic molecules to show how many carbon atoms are bonded together successively (in a ‘chain’).
meth = 1 carbon, eth = 2 carbons, pro = 3 carbons, but = 4 carbons, pent = 5 carbons, hex = 6 carbons, hept = 7 carbons, oct = 8 carbons, non = 9 carbons, dec = 10 carbons
Hydrocarbons are molecules made up of only carbon and hydrogen atoms.
- Alkanes have single bonds between each carbon atom, all other available bonds are made to hydrogen atoms – they are called ‘saturated’ hydrocarbons.
- Alkenes have a double bond between two of their carbon atoms – they are called ‘unsaturated’ hydrocarbons.
Alkly groups are carbon chain groups bonded to another carbon chain.

Functional Groups

Alkane

Alkene

Halogenoalkane

Alcohol

Aldehyde

Ketone

Carboxylic Acid

Structural Isomerism

Molecules that have the same molecular formula but different structures are called structural isomers.
Chain isomers have different carbon chain arrangements to one another.
Positional isomers have a functional group in different positions on their carbon chains.

Nomenclature

Nomenclature is the process of naming compounds in organic chemistry.
To name a compound, four basic rules are followed:
- Step 1 Identify the of longest carbon chain and choose the prefix (meth, eth...).
- Step 2 Identify the functional groups and choose the suffix (-ol, al…).
- Step 3 Identify the position of functional groups on the carbon chain (-1-ol, -3-ene.).
- Step 4 Place functional groups and alkyl chains in alphabetical order – if more than one.

Alkanes

Alkanes are hydrocarbons in which the carbon atoms are bonded together with single covalent bonds.
Alkanes are non-polar and do not dissolve in water or polar solvents.
The combustion of alkanes releases large amounts of energy, making alkanes useful as fuels.
- Complete combustion of alkanes releases carbon dioxide, incomplete combustion releases carbon monoxide.
Short chain hydrocarbons have low melting and boiling points (due to fewer intermolecular forces holding molecules together).
Long chain hydrocarbons have high melting and boiling points (due to greater intermolecular forces holding molecules together).

Alkanes - Free Radical Substitution

Covalent bonds can break in two ways:
- Heterolytic fission - bond breaks unevenly and both electrons from the bond go to one atom.
- Homolytic fission - bond breaks evenly and each bonded atom gets one electron, forming free-radicals.
  - Free radicals are species that have an unpaired electron and are highly reactive.
- Halogens can react with alkanes in free-radical substitution reactions, the mechanism occurs as a ‘chain’ reaction with three stages:
  - Initiation - U.V. light is needed to start the reaction and cause homolytic fission of the halogen molecule, creating two halogen radicals.
  - Propagation – radical species react with the alkane and get substituted into the molecule, creating further radicals.
  - Termination – two radical species combine to create a covalent bond and terminate the chain as the product is not a free radical.

Alkenes

Alkenes are hydrocarbons in which two or more of the carbon atoms are bonded together with a double bond.
A carbon double bond is made by the merging of a 2p-orbital from each carbon atom, creating an area of high electron density between the two atoms, called a ‘pi-bond’.
Electron deficient species (electrophiles) are attracted to the electrons in the double bond, which makes alkenes more reactive than simple alkanes.

Alkenes - Stereoisomerism

Carbon double bonds are unable to rotate freely like single bonds do – they have restricted rotation.
Groups or atoms bonded to carbon atoms in a double bond are ‘locked’ into position, and there are two possible ways they can be arranged.
Stereoisomerism occurs when two molecules have the same molecular and structural formula, but atoms within the molecules are arranged in space differently.
Z and E notation is used to name alkene based stereoisomers.
- In Z isomers, the highest priority groups bonded to each carbon in the double bond are pointing in the same direction.
- In E isomers, the highest priority groups bonded to each carbon in the double bond are pointing in opposite directions.
Cis and trans isomers are forms of Z and E isomers (respectively), but both carbons in the double bond are bonded to the same type of groups.

Alkenes - Electrophilic Addition Reactions

Alkenes react by electrophilic addition reactions.
- Electron deficient species (electrophiles) are attracted to the pi-bonded electrons in a carbon double bond.
- In electrophilic addition, an electrophile causes the double carbon bond to break, and a new bond is formed between the electrophile and one of the carbon atoms.
A carbocation (positively charged carbon atom) intermediate is formed that a negatively charged species forms a bond with.
- Primary carbocations are less stable than secondary and tertiary carbocations because they experience less of an inductive effect, meaning they aren’t as likely to form during electrophilic addition.
Major and minor products of electrophilic addition reactions are determined by the stability of the intermediate carbocation that forms.

Alcohols

Alcohols are hydrocarbons with a hydroxyl (OH) group bonded to a carbon in the chain.
The O-H bond in alcohols is highly polar, meaning short chain alcohols (methanol and ethanol) are soluble in water.
Longer chain alcohols are insoluble in water as the carbon chain (alkyl) is not polar.
Hydrogen bonds can form between alcohol molecules, giving them higher melting and boiling points compared to alkanes with the same carbon chain.
Alcohols can be primary (OH group bonded to a carbon bonded to only one other carbon), secondary (OH group bonded to a carbon bonded to two other carbon atoms) and tertiary (OH group bonded to a carbon bonded to three other carbon atoms).

Oxidation of Alcohols

In organic chemistry, oxidation is a carbon atom gaining a bond to an oxygen atom and/or losing a bond to a hydrogen atom.
To oxidise an alcohol, an oxidising agent (usually acidified potassium dichromate) is used and the alcohol is heated.
Primary alcohols can be oxidised to an aldehyde, then to a carboxylic acid.
- To isolate the aldehyde, the products must be distilled from the reaction mixture.
- If a carboxylic acid is desired, the mixture must be heated under reflux conditions.
Secondary alcohols can only be oxidised to form ketones.
Fehling’s solution and Tollens’ reagent are used to distinguish between aldehydes and ketones.
- Aldehydes react to form a brick red precipitate with Fehling’s solution and a silver solid (silver mirror) with Tollens’ reagent. Ketones do not react with either.
Teritary alcohols cannot be oxidised.

Halogenoalkanes

Halogenoalkanes - Nucleophilic Substitution Reactions

Carbon-halogen bonds are highly polar.
- Due to the high electronegativity of halogens, the carbon becomes partially positively charged and the halogen partially negatively charged.
Polarity of the carbon-halogen bonds decreases as you go down group 7.
The carbon in a carbon-halogen bond is easily attacked by electron donating species (nucleophiles) that swap places with the halogen in nucleophilic substitution reactions.
Key Reactions:
- Halogenoalkanes with sodium hydroxide in aqueous conditions form alcohols.
- Halogenoalkanes with ammonia in ethanolic conditions form amines.
- Halogenoalkanes with cyanide ions in ethanolic conditions form nitriles.
  - Ethanolic conditions are needed instead of aqueous conditions, otherwise alcohols would form.

Halogenoalkanes - Elimination Reactions

Halogenoalkanes can be converted to alkenes in an elimination reaction.
By reacting halogenoalkanes with hydroxide ions in ethanolic conditions (anhydrous), an alkene and not an alcohol is formed.
The reaction is carried out under reflux conditions and the hydroxide ion acts as a base (unlike in the hydrolysis of a halogenoalkane to form an alcohol).