All alcohols contain the hydroxyl functional group. Subsequently, all alcohols have similar chemical properties. The properties of alcohols are as follows:
- structure of alcohols is similar to that of water. Tje bond angles of water molecules are the same as the bond angle of the hydroxyl group in the alcohol. This means the functional group has a dipole moment
- Alcohols can form hydrogen bonds. This means alcohols have higher boiling points than other chemicals. For example, between butane there would be van der waals forces, but between butanol molecules there are also hydrogen bonds.
- The longer an alcohol chain is, the less soluble it is in polar compounds. This is because the influence of the electronegative OH functional group is less obvious.
- However, the longer the chain is the more soluble it is in non-polar compounds. This is because the longer chains are less polar.
Types of alcohol
Sometimes alcohols will have alkyl groups attached to the central carbon (the carbon which the OH functional group is attached to). These alkyls consist of one carbon atom and three hydrogen atoms. This is represented as CH3.
- Primary alcohols: If an alcohol has one of these alkyl groups, it is a primary alcohol. If an alcohol has none, it is also primary.
- Secondary alcohols:If an alcohol has two alkyl groups attached to the central carbon atom, it is a secondary alcohol.
- Tertiary alcohols:If an alcohol has three alkyl groups attached to the central carbon, it is a tertiary alcohol.
Essentially, the different alcohols look like this:
Oxidation of primary alcohols
Oxidation of alcohols
The diagrams below summarise the oxidation of the different types of alcohol:
How Primary alcohols are oxidised
Distilation: This is used to partiarly oxidise primary alcohols into aldehydes.
- An excess of alcohol is mixed with insufficent oxidising agent.
- The mixture is vapourised which allows the aldehyde to rise into the condenser
- The aldehyde is collected at the end. The liquid in the distilation flask turns from organe to green.
Reflux: Primary alcohols are oxidised in reflux conditions so they oxidise fully into carboxylic acids
- The alcohol is mixed with an excess of oxidising agent
- Mixture is vapourised. The vapour rises, condenses and falls back down. The process continues.
- The solution in the flask turns from orange to green when oxidisation is complete.
And ester is derived from a carboxylic acid and an alcohol. Esters have several uses, such as polymers, solvents (examples include nail polish remover, paint and glue), and in cosmetics.
The process happens when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The process is slow and reversible. An example is shown below.
Alcohols can be dehydrated in the prescence of an acid catalyst to produce an alkene.
Halogenoalkanes are alkanes which contain halogens
Fluorine is the most electronegative element. As a result the dipoles created in the polar bonds are very strong. Down the group the strength of these dipoles decreases. It is said that the polarity of the bond decreases. As a result the bonds are less reactive. This happens for several reasons:
- Down the group there is more electron shielding
- The atomic radius increases so the attraction between the halogenoalkane and other compounds decreases
- These two factors cancel out the increasing density of the positive nuclear charge.
Bond enthalpies are the strengths of the carbon-halogen bonds. The bond enthalpies decrease down group 7. Bonds with a lower enthalpy are weaker, and less energy is required to break these bonds. Subsequently the rate of the reaction increases. The rate of reaction increases down group seven.
This is where an atom or group of atoms is replaced by an electron donor known as a nucleophile.
The hydroxide ion is attracted to the electron deficient carbon atom, as the shared pair of electrons are nearer the halogen as it is more electronegative.
hydroxide ion has a lone pair of electrons which it donates to the carbon to form a dative covalent bond, replacing the covalent bond between the halogen and the carbon.
- The C-Halogen bond is broken through heterolytic fission.. This means that carbon becomes positively charged, but receives electrons form the hydroxide ion and so loses this charge, while the halogen becomes a negative ion.
These are types of halogenoalkanes. The CFCs are very stable due to their Carbon-halogen bonds. They are also very useful as they are non-toxic and are non-flammable.
However, CFCs are broken down by ultraviolet radiation when they reach the stratosphere to form chlorine radicals. This causes ozone depletion
As shown in the diagram, the free radical subtition removes ozone from the atmosphere.
Alternatives to CFCs
Alternatives to CFCs
- were the first substances used as replacements for CFCs.
- The carbon-hydrogen (C-H) bonds in HCFCs are susceptible to attack by reactive radicals and atoms in the troposphere, and are therefore decomposed there to a significant extent.
- However, they still diffuse into the stratosphere and cause significant ozone destruction.
- Don't contain chlorine so do not form chlorine radicals
- Firdges use ammonia as coolant gas
- Carbon dixoide used to make foamed polymers
- Aerosols use oump spray systems or nitrogen as propellant.
Polytetrafluoroethene which is known as PTFE is a polymer which contains a halogen. It is known as Teflon. The carbon-fluorine bonds in PTFE are very strong. As a result PTFE is resistant to chemical attack and is inert (unreactive). The polymer is also heat resistant, has non-stick properties and is a good electrical insulator. As a result the polymer is used to give pans their non-stick coating.
Polyvinylchloride is another halogen containing polymer. PVC is used in drain pipes and sports equipment. It is formed from the polymerisation of chloroethene.