4.2. Alcohols, Haloalkanes and Analysis: Module 4: Core Organic Chemistry: Chemistry OCR A A Level

Alcohols

Alcohols contain an -OH group and follow the general formula CnH2n+1OH. They can be produced via two main methods of fermentation or hydration.

Alcohols are named according to IUPAC rules and have the suffix -ol.

Alcohols can be primary (1o), secondary (2o) or tertiary (3o), depending on the position of the hydroxyl group.

Hydrogen Bonds

Hydrogen bonds are a much stronger intermolecular force than van der Waals forces so more energy is required to overcome them.

Combustion

When burned in air, alcohols react with oxygen to form carbon dioxide and water. Alcohols make good fuels by reacting in this way as lots of energy is also released.

PCl5

PCl5 is used to produce chloroalkanes.

Elimination Reactions

Alkenes can be formed from the dehydration of alcohols, where a molecule of water is removed from the molecule. In order to do this, the alcohol is heated with concentrated phosphoric acid.

Oxidation of Alcohols

Primary and secondary alcohols can be oxidised to produce various products but tertiary alcohols are not easily oxidised.

Nucleophiles

A nucleophile is an electron pair donor. These species are ‘positive liking’. They contain a lone electron pair that is attracted to ∂+ regions of molecules.

Some of the most common nucleophiles.

● CN:-

● :NH3

● -:OH

Nucleophilic Substitution Mechanism

Nucleophilic substitution is the reaction mechanism that shows how nucleophiles attack haloalkanes. Aqueous potassium hydroxide is used to produce alcohols, potassium cyanide is used to produce nitriles and ammonia is used to produce amines.

Heating under Reflux

Reflux apparatus is used to continually heat the contents of the flask to allow reactions like the oxidation of primary alcohols to proceed all the way to the formation of carboxylic acids. The condenser helps ensure the vapours condense and return to the flask for further heating. This ensures the product vapours can not escape.

Distillation

Distillation apparatus is used to separate liquids with different boiling points. The round-bottomed flask is heated and the liquid with the lower boiling point will evaporate first. It rises out of the flask and into the attached tubing which is surrounded by a condenser. The condenser causes the vapour to cool and condense back into a liquid, which is then collected in a separate flask.

Separating Funnel

A separating funnel is used to separate two liquids with different densities.

Redistillation

Subsequent distillations can be carried out to obtain a purer product.

Drying

A compound can be dried by the addition of an anhydrous (contains no water) salt. The anhydrous salt will absorb moisture and water present, thus drying and purifying the compound. Common anhydrous salts used for drying are magnesium sulphate and calcium chloride.

Synthetic Routes

Synthetic routes are the routes which can be used to produce a certain product from a starting organic compound. It is important that you understand the different methods and conditions required to convert compounds to other products.

Reagents

Renewable reagents with few safety concerns are preferred.

By-products

Less harmful by-products are favoured as there would be fewer safety and environmental concerns. If the by-products can be used in another industry, the process is more sustainable.

Conditions

Choose the reaction with the most energy efficient and safe conditions.

Mass Spectrometry

Mass spectrometry is an analytical technique used to identify different molecules and find the overall relative molecular mass.

Ionisation

A sample is vaporised and injected into the mass spectrometer where a high voltage is passed over the chamber. This causes electrons to be removed from the atoms (they are ionised) leaving +1 charged ions in the chamber.

Acceleration

These positively charged ions are then accelerated towards a negatively charged detection plate.

Ion Drift

The ions are then deflected by a magnetic field into a curved path. The radius of the path is dependent on the charge and mass of the ion.

Detection

When the positive ions hit the negatively charged detection plate, they gain an electron, producing a flow of charge. The greater the current produced, the greater the abundance of that particular ion.

Analysis

The current values are then used in combination with the flight times to produce a spectra print-out with the relative abundance of each isotope displayed.