Halogneoalkanes

Halogenoalkanes = an alkane with at least one halogen atom instead of a hydrogen atom 

HOW TO NAME: 1.Identify longest carbon chain 

                             2. Put halogens in alphabetical order 

                             3. Number halogens 

n.b. if more than one identical halogen use di, tri, tetra before the halogen name 

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Boiling points and polarity

BOILING POINTS: Increase down the group (same as halogens)

Strength depends on intermolecular forces and as go down G7 atomic radius increases meaning the number of shells increase. Therefore, stronger id-id to overcome which requires more energy.

POLARITY: Halogens more electronegative than carbon, so therefore C-X bond polar. 

Partially +ve Carbon lacks e- and so can be attacked by a nucelophile -> a nucleophile is an e- pair donor e.g. :OH, :NH3, H2O:

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Naming

Halogenoalkane = an alkane with at least one halogen atom instead of a hydrogen atom 

NAMING: 1. Identify longest carbon chain 

                  2. Put the halogens in alphabetical order 

                  3. number those halogens 

n.b. if more than one identical halogen put di, tri, tetra in front of halogen name 

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Making chloroalkanes

Alcohol (usually tertiary) + conc.HCL -> Impure chloroalkane + H2O

1. THE REACTION 

shake the tertiary alcohol with conc HCL in a separating funnel (as product is volatile) for 20 mins 

2. SEPARATION

let the mixture settle into layers and run off the aqueous lower layer which leaves impure halogenoalkane 

3.PURIFICATION

- neutralise excess acid - add NaOH until no more gas is produced, then run lower layer off

- add distilled water, shake and run off again - this removes any inorganic impurities 

- water removed by adding anhydrous sodium sulphate and shaking 

- remaining impurities are removed by distillation - chloroalkane boil between 48 and 53*C

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Nucleophillic substitution of halogenoalkanes

OH = nucleophile 

warm aqueous NaOH

partially +ve C attracts lone pair of e- on :OH. The C-Br bond breaks heterolytically (Br gains both e- from OH) and new bond forms between C - OH 

Water = nucleophile 

partially +ve C attracts lone pair of e- on :OH. Unstable intermediate formed as O2 has X3 lone pairs, one OH bond breaks. Alcohol formed 

Ammonia =  nucleophile 

Warm halogenoalkane with ammmonia dissolved in ethanol 

Reflux halogenoalkane with ammonia -> ammonia attaches to the alkane and Br substituted. Ammonia then removes a H from the NH3 leaving an amine and NH4+.

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