AS-Level Organic

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 structure of a molecule.

  • Skeletal formula shows the carbon backbone and functional groups within molecules, no hydrogen bonds are shown.

Carbon Chains

  • Prefixes are used when naming organic molecules to show how many carbon atoms are 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 and dec = 10 carbons 

Isomerism (Structural)

  • Molecules that have the same molecular formula but different structures are called structural isomers.

  • Chain isomers have different carbon chains.

  • Positional isomers have a functional group in different positions on a carbon chain.

Nomenclature

  • Nomenclature is the process of naming compounds in organic chemistry.

  • To name a compound, four basic rules are followed:

    • Identify the of longest carbon chain and choose the prefix (meth, eth..).

    • Identify the functional groups and choose the suffix (-ol, al…).

    • Identify the position of functional groups on the carbon chain (-1-ol, -3-ene.).

    • Place functional groups and alkyl chains in alphabetical order, if more than one.

AS-Level Alkanes

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).

Free Radical Substitution

  • Covalent bonds can break in two ways

    • Heterolytic fission: bond breaks unevenly and both bonded electrons go to one atom, creating positive and negative ions.

    • Homolytic fission: bond breaks evenly and each bonded atom get one electron, forming free-radicals.

  • Halogens can react with alkanes in free-radical substitution, 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.

AS-Level Alkenes

Alkenes

  • Alkanes 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 two p-orbitals from two carbon atoms, 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 and this makes alkenes more reactive than simple alkanes.

Stereoisomerism (E and Z)

  • Carbon double bonds are unable to rotate freely like single bonds, 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.

  • 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, but both carbons in the double bond are bonded to the same type of groups.

Electrophilic Addition of Alkenes

  • Electron pair acceptors (electrophiles) are attracted to the pi-bonded electrons in a carbon double bond and react through electrophilic addition reactions with an alkene.

  • 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 carbons.

    • Alkene + Bromine → Dibromo-alkene 

    • Alkene + Hydrogen Bromide → Bromo-alkene 

  • A carbocation (contains 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, as they experience less of an inductive effect, meaning they are less 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.

AS-Level Alcohols

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 number of carbons. 

  • 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

  • For organic chemistry – oxidation is a carbon atom gaining a bond to an oxygen atom 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 and 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. Adehydes 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.

AS-Level Halogenoalkane

Nucleophilic Substitution of Halogenoalkanes

  • Carbon-halogen bonds are highly polar.

  • Due to the high electronegativity of the halogens, the carbon becomes slightly positively charged and the halogen slightly 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 forms alcohols.

    • Halogenoalkanes with ammonia in ethanolic conditions forms amines.

    • Halogenoalkanes with cyanide ions in ethanolic conditions forms nitriles.

    • ​Ethanolic conditions are needed instead of aqueous conditions otherwise alcohols would form.
       

Elimination Reactions of Halogenoalkanes

  • 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 and the hydroxide ion acts as a base (unlike in the hydrolysis of a halogenoalkane to form an alcohol).