AS-Level Organic
Introduction
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Empirical formula shows the simplest whole number ratio of elements within a molecule.
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Molecular formula shows the actual number of atoms of each element in a molecule.
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Structural formula shows how the atoms are arranged in a molecule.
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Displayed formula shows a drawing of the structure of a molecule.
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Skeletal formula shows the carbon backbone and functional groups within molecules, no hydrogen bonds are shown.
Carbon Chains
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Prefixes are used when naming organic molecules to show how many carbon atoms are in a chain.
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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)
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Molecules that have the same molecular formula but different structures are called structural isomers.
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Chain isomers have different carbon chains.
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Positional isomers have a functional group in different positions on a carbon chain.
Nomenclature
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Nomenclature is the process of naming compounds in organic chemistry.
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To name a compound, four basic rules are followed:
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Identify the of longest carbon chain and choose the prefix (meth, eth..).
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Identify the functional groups and choose the suffix (-ol, al…).
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Identify the position of functional groups on the carbon chain (-1-ol, -3-ene.).
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Place functional groups and alkyl chains in alphabetical order, if more than one.
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AS-Level Alkanes
Alkanes
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Alkanes are hydrocarbons in which the carbon atoms are bonded together with single covalent bonds.
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Alkanes are non-polar and do not dissolve in water or polar solvents.
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The combustion of alkanes releases large amounts of energy, making alkanes useful as fuels.
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Complete combustion of alkanes releases carbon dioxide, incomplete combustion releases carbon monoxide.
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Short chain hydrocarbons have low melting and boiling points (due to fewer intermolecular forces holding molecules together).
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Long chain hydrocarbons have high melting and boiling points (due to greater intermolecular forces holding molecules together).
Free Radical Substitution
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Covalent bonds can break in two ways
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Heterolytic fission: bond breaks unevenly and both bonded electrons go to one atom, creating positive and negative ions.
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Homolytic fission: bond breaks evenly and each bonded atom get one electron, forming free-radicals.
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Halogens can react with alkanes in free-radical substitution, the mechanism occurs as a ‘chain’ reaction with three stages
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Initiation - U.V. light is needed to start the reaction and cause homolytic fission of the halogen molecule, creating two halogen radicals.
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Propagation – radical species react with the alkane and get substituted into the molecule, creating further radicals.
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Termination – two radical species combine to create a covalent bond and terminate the chain as the product is not a free radical.
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AS-Level Alkenes
Alkenes
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Alkanes are hydrocarbons in which two or more of the carbon atoms are bonded together with a double bond.
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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’.
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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)
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Carbon double bonds are unable to rotate freely like single bonds, they have restricted rotation
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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.
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Stereoisomerism occurs when two molecules have the same molecular and structural formula, but atoms within the molecules are arranged in space differently.
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In Z-isomers, the highest priority groups bonded to each carbon in the double bond are pointing in the same direction.
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In E-isomers, the highest priority groups bonded to each carbon in the double bond are pointing in opposite directions.
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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
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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.
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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.
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Alkene + Bromine → Dibromo-alkene
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Alkene + Hydrogen Bromide → Bromo-alkene
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A carbocation (contains positively charged carbon atom) intermediate is formed that a negatively charged species forms a bond with.
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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.
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Major and minor products of electrophilic addition reactions are determined by the stability of the intermediate carbocation that forms.
AS-Level Alcohols
Alcohols
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Alcohols are hydrocarbons with a hydroxyl (OH) group bonded to a carbon in the chain.
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The O-H bond in alcohols is highly polar, meaning short chain alcohols (methanol and ethanol) are soluble in water.
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Longer chain alcohols are insoluble in water as the carbon chain (alkyl) is not polar.
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Hydrogen bonds can form between alcohol molecules, giving them higher melting and boiling points compared to alkanes with the same number of carbons.
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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
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For organic chemistry – oxidation is a carbon atom gaining a bond to an oxygen atom or losing a bond to a hydrogen atom.
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To oxidise an alcohol an oxidising agent (usually acidified potassium dichromate) is used and the alcohol is heated.
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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.
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Secondary alcohols can only be oxidised to form ketones.
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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
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Carbon-halogen bonds are highly polar.
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Due to the high electronegativity of the halogens, the carbon becomes slightly positively charged and the halogen slightly negatively charged.
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Polarity of the carbon-halogen bonds decreases as you go down group 7.
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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
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Halogenoalkanes with sodium hydroxide in aqueous conditions forms alcohols.
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Halogenoalkanes with ammonia in ethanolic conditions forms amines.
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Halogenoalkanes with cyanide ions in ethanolic conditions forms nitriles.
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Ethanolic conditions are needed instead of aqueous conditions otherwise alcohols would form.
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Elimination Reactions of Halogenoalkanes
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Halogenoalkanes can be converted to alkenes in an elimination reaction.
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By reacting halogenoalkanes with hydroxide ions in ethanolic conditions (anhydrous), an alkene and not an alcohol is formed.
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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).