Chemistry 6.2 - Halogenoalkanes, Alcohols & Modern Analytical Techniques I

Halogenoalkanes are alkanes with at least one halogen atom in place of a hydrogen atom. There are three classes based on how many alkyl groups the carbon atom the halogen is attached to has:

Primary - Two hydrogen atoms and one alkyl group

Secondary - One hydrogen atom and two alkyl groups

Tertiary - No hydrogen atoms but three alkyl groups

Halogenoalkanes can undergo several types of reactions:

They can be hydrolysed to form alcohols in a nucleophilic substitution reaction. One way to do this is to use water.

R-X + H₂O --> R-OH + H⁺ + X^-

E.g. with bromoethane: CH₃CH₂Br + H₂O --> C₂H₅OH + H⁺ + Br^-

Halogenoalkanes can react with Aqueous KOH to form alcohols

R-X + KOH --> ROH + KX

Cyanide ions react with halogenoalkanes to form nitriles.

R-X + CN^- --> R-C≡N + X^-

The halogenoalkane is refluxed and has to be done in ethanol.

Halogenoalkanes can react with Ammonia to form Amines

- Warm a halogenoalkane in excess ethanolic ammonia. The ammonia swaps places with a halogen to form a primary amine.

e.g. C₂H₅Br + NH₃ --> CH₃CH₂NH₃ + Br^-

Halogenoalkanes can also undergo elimination reactions by refluxing it in a warm alkali dissolved in ethanol.

e.g. C₂H₅Br + KOH --> C₂H₄ + H₂O + KBr

Halogenoalkanes can be hydrolised to form alcohols via a nucleophilic substitution reaction in water.

R-X + H₂O --> R-OH + H⁺ + X^-

In an experiment, a precipitate will form if silver nitrate is added. The reaction is a silver halide.

Chloro - White precipitate

Bromo - Cream precipitate

Iodo - Yellow precipitate 

Primary, secondary and tertiary halogenoalkes will have different reactivities. A tertiary halogenoalkane is the most reactive and the primary halogenoalkane is the least reactive.

In order to hydrolise a halogenoalkane, you need to break the carbon halogen bond. Different halogens will take longer to react depending on its size. The size of the halogen increases down group 7 so iodoalkanes have weak bonds so are hydrolised the fastest. Fluoroalkanes have the strongest bonds so are the slowest.

Alcohols are organic compounds with the general formula C_nH_2n+1OH

Alcohols are divided into three classes based on how many alkyl groups the carbon the OH group is attached to has:

Primary - One alkyl  group

Secondary - Two alkyl groups

Tertiary - Three alkyl groups

Alcohols can react in substitution reactions to form halogenoalkanes.

Reacting with PCl5 or HCl produces chloroalklanes

ROH + PCl₅ --> RCl + HCl + POCl₃

ROH + HCl + RCl + H₂O

The -OH can be swapped for bromine to make a bromoalkane. This reaction requires an acid catalyst.

Iodoalkanes can be made using red phosphorus and iodine.

3ROH + PI3 + 3RI + H3PO3

Alcohols can be dehydratred to form alkenes by removing water from alcohols in an elimination reaction. This reaction requires an acid catalyst and is then heated.

E.g. C₂H₅OH --> CH₂CH₂ + H₂O

Both aldehydes and ketones have the same functional group (C=O). The difference between them is where the functional group is:

Aldehydes have a hydrogen and one alkyl group attached to the carbon atom. e.g. Propanal (CH₃CH₂CHO)

Ketones have two alkyl groups attached to the carbon atom. e.g. Propanone (CH₃COCH₃)

You can test whether a substance is an aldehyde or a ketone using Benedict's solution (a blue solution of complexed copper(II) ions in sodium carbonate). If heated with an aldehyde, the ions are reduced to a brick red precipitate. On the other hand, nothing happens if you heat benedict's with a ketone because it cannot be easily oxidised.

The same test can be done using Fehling's solution in which aldehydes turns from blue to red and a ketone does nothing.

The simplest way to oxidise an alcohol is to make them undergo a complete combustion reaction.

C₂H₅OH_(l) + 3O_2(g) --> 2CO_2(g) 3H₂O_(g)

How much an alcohol can be oxidies depends on its structure:

Primary alcohols are oxidised to form aldehydes and then carboxylic acids

Primary alcohol - Distillation -> Aldehyde - Reflux -> Carboxylic acid

Secondary alcohols are oxidised to ketones

Secondary alcohol - Reflux -> Ketone

Tertiary alcohols can't be oxidised. The only way to do so is by burning them.