Topic 6 halogenoalkanes

what is an halogenoalkane?

an alkane with at least one halogen atom in place of a hydrogen atom

what is a primary secondary or tertiary halogenoalkane?

On the carbon with the halogen attached:

1) A primary halogenoalkane has two hydrogen atoms and just one alkyl group.

2) A secondary halogenoalkane has just one hydrogen atom and two alkyl groups.

3) A tertiary halogenoalkane has no hydrogen atoms and three alkyl groups

halogenoalkanes can be hydrolyzed to form what? and how?

1.Halogenoalkanes can be hydrolyzed to alcohols in a nucleophilic substitution reaction

2. can be done by heating with water ,products are alcohol and hydrogen halide

3.water is a weak nucleophile so the reaction is slower than using a hydroxide

(4. also with aqueous potassium hydroxide)

equation for bromoethane being hydrolyzed to from alcohol using water

CH3 CH2 Br + H2 O - C2 H5 OH + H+ + Br-

how to compare reactivity of halogenoalkanes using experiment

1) halogenoalkanes react with water to form alcohols

2) If you put silver nitrate solution (AgNO3) in the mixture too, the silver ions react with the halide ions as soon as they form, giving a silver halide precipitate

3) To compare the reactivities of different halogenoalkanes, set up three test tubes each containing a different halogenoalkane, ethanol (as a solvent) and silver nitrate solution (this contains the water)

4) Time how long it takes for a precipitate to form in each test tube. The more quickly a precipitate forms, the faster the rate of hydrolysis is for that halogenoalkane.

what is the trend in reactivity of halogenoalkanes?

more reactive as we go down the group and hydrolyze the fastest

tertiary>secondary>primary

what bond has to break in order to hydrolyse a halogenoalkane

carbon halogen bond

what depends how quickly halogenoalkanes hydrolyse?

-carbon halogen bond enthalpy

-Bond enthalpy depends on the size of the halogen — the larger the halogen, the longer the C-X bond, and the lower the bond enthalpy

-so weaker carbon halogen bonds breaks easily

-so they react faster

which halogenoalkanes hydrolyses the fastest and slowest?

1.fastest: iodoalkanes have the weakest bonds so is hydrolyzed fastest

2.fuoroalkane has the strongest bonds so they are slowest at hydrolysing

what color is the precipitate formed when iodoalkane , chloroalkane and bromoalkene react?

1.iodoalkane: yellow precipitate

2.bromoalkane: cream precipitate

3.choloralkane: white precipitate

why is a carbon halogen bond polar?

The halogens are more electronegative than carbon.

What is a nucleophile and give examples

-electron pair donors

-OH-, NH3 , CN-

-water is a weak nuclophile so the reaction with water is slower

halogenoalkanes react with warm aqueous KOH/NaOH under reflux to form..(aqueous = substitution)? what type of reaction is this?

alcohols

nucleophilic substitution reaction

draw the reaction mechanism for bromoethane reacting with hydroxide to form ethanol and write equation

how does halogenoalkanes react with cyanide ions to make nitriles

1.conditions:-

-warm ethanolic pottasium cynaide

-carried under reflux

2.The cyanide ion (CN-) acts as the nucleophile, displacing the halogen to form a nitrile product.

3. carbon chain increases by one

draw the reaction mechanism for bromoethane reacts with cyanide to form ethanenitrile and write equation

what's the difference between an amine molecule and a ammonia molecule

amines are organic compounds that are based on ammonia but has their one or more hydrogen atoms replaced by alkyl groups

whats happening when halogenoalkanes react with ammonia to produce primary amines

-warm a halogenoalkane with excess ethanolic ammonia (acts as a nucleophile) to form a primary amine by nucleophilic substitution

-an intermediate is formed (NH3+)

-another molecule of NH3 acts as a base (bases are proton acceptors) and reacts with the hydrogen (reason for excess ammonia)

-electrons in N-H bonds moves to the N from H

-an amine is produced alongside NH4+ and halogen ion

draw the reaction mechanism for bromoethane reacting with ammonia to form ethylamine and write equation

explain how halogenoalkanes undergo elimination reactions with a warm alkali dissolved in ethanol(alcoholic = elimination)?

-warm ethanolic potassium/sodium hydroxide

-carried out under reflux

-OH- will attack the hydrogen on a carbon adjacent to the carbon with a halogen attached

-OH- acts as a base forming water

-the electrons in the bonds moves to form a double bond between two carbons

-C-X bond breaks as the the electrons from the bond moves to the X atom

draw the reaction mechanism for the reaction of 2-iodopropane with KOH giving propene. write full equation