Halagenoalkane

Question 1

what is a primary halogenoalkane

Answer 1

when a halogen is attached to a carbon that itself is attached to one other alkyl group

Question 2

what is a secondary halogenoalkane

Answer 2

when a halogen is attached to a carbon that itself is attached to two other alkyl groups

Question 3

what is a nucleophile

Answer 3

electron-rich species that can donate a pair of electrons

Question 4

how are alcohols formed

Answer 4

aqueous sodium or potassium hydroxide, to produce alcohols in a nucleophilic substitution reaction. The hydroxide ion acts as a nucleophile. This is an example of a hydrolysis reaction

Question 5

how does the shortness of the bond and Mr have an impact on the rate of reaction

Answer 5

Shorter bonds are stronger because the atoms are held together more tightly, making the bond harder to break. As bonds become shorter, the attraction between atoms grows stronger requiring more energy to pull them apart.

The greater the Mr of the halogen in the polar bond, the lower the bond enthalpy. A lower bond enthalpy means the bond can be broken more easily. Therefore, the rate of reaction increases for halogenoalkanes as you move down the group.

Question 6

reaction with water

Answer 6

The water molecule is a weak nucleophile, but it will eventually substitute for the halogen
This occurs much more slowly compared to when warm aqueous sodium hydroxide is used
An alcohol is produced
CH3CH2Br + H2O → CH3CH2OH + H+ + Br-

Question 7

why is hydroxide a better nucleophile

Answer 7

as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge; this causes the nucleophilic substitution reaction with water to be much slower than the aqueous alkali

Question 8

how can you test it

Answer 8

If silver nitrate solution in ethanol is added to the solution, the silver ions will react with the halide ions as soon as they form, giving a silver halide precipitate.

Question 9

how can form nitriles

Answer 9

react with ethanolic potassium cyanide (KCN) to form nitriles in a nucleophilic substitution reaction. The cyanide ion, CN, acts as a nucleophile. This reaction adds on a carbon atom, so it can be used in synthesis routes to increase the length of carbon chains. Forms nitrile and halide

Question 10

how are amines formed

Answer 10

An ethanolic solution of excess ammonia (NH3 in ethanol) is heated under pressure with a primary halogenoalkane
An excess of ammonia is used because the product is more reactive than ammonia so further substitution reactions could occur
The product is a primary amine and HX

Question 11

how are alkenes formed

Answer 11

The halogenoalkanes are heated with ethanolic sodium hydroxide causing the C-X bond to break heterolytically, forming an X- ion and leaving an alkene as an organic product. Formed alkene water and NaX

Question 12

what is a nucleophilic substitution

Answer 12

a nucleophile attacks a carbon atom which carries a partial positive charge
An atom that has a partial negative charge is replaced by the nucleophile
Halogenoalkanes will undergo nucleophilic substitution reactions due to the polar C-X bond

Question 13

how is ammonia chloride and methylamine formed

Answer 13

Excess ammonia is used to prevent further substitution and favour the formation of a primary amine. NH3 attaches to hydrogen on N-H bond to break it.

Question 14

what can be used to measure the rate of hydrolysis of halogenoalkanes

Answer 14

Acidified aqueous silver nitrate

Question 15

what is the method used to measure rate of hydrolysis of halogenoalkanes

Answer 15

Set up three test tubes in a 50 oC water bath, with a mixture of ethanol and acidified silver nitrate

Add a few drops of a chloroalkane, bromoalkane and an iodoalkane to each test tube and start a stop watch

Time how long it takes for the precipitates to form

Question 16

which halogenoalkane is most and least reactive

Answer 16

The yellow silver iodide precipitate is the fastest nucleophilic substitution reaction
This is because the C-I bond has the lowest bond enthalpy, so it is the easiest to break

The white chloride precipitate is the slowest nucleophilic substitution reaction
This is because the C-Cl bond has the highest bond enthalpy, so it is the hardest to break and will cause the Cl- ions to form the slowest

Question 17

what about silver fluoride

Answer 17

Silver fluoride is soluble, so a precipitate will not be formed in this reaction

Question 18

what are the two ways nucleophilic substitution can occur

Answer 18

SN1 reactions - Tertiary halogenoalkanes
SN2 reactions - Primary halogenoalkanes

Question 19

why is tertiary more reactive and faster than primary

Answer 19

A primary halogenoalkane has only one alkyl group attached to the carbon, so it is difficult for the halide ion to escape, hence the reaction is slowest.

Question 20

what does 1 and 2 stand for

Answer 20

the rate of the reaction (which is determined by the slowest step of the reaction) depends on the concentration of only one reagent, the halogenoalkane

Question 21

explain the Sn1 mechanism

Answer 21

In the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X- ion (this is the slow and rate-determining step)
This forms a tertiary carbocation (which is a tertiary carbon atom with a positive charge)
In the second step, the tertiary carbocation is attacked by the nucleophile

Question 22

explain sn2 reactions

Answer 22

In primary halogenoalkanes, the carbon that is attached to the halogen is bonded to one alkyl group

The SN2 mechanism is a one-step reaction
The nucleophile donates a pair of electrons to the δ+ carbon atom of the halogenoalkane to form a new bond
the C-X bond is breaking and the halogen (X) takes both electrons in the bond
The halogen leaves the halogenoalkane as an X- ion

Question 23

what would you add to dissolve precipitate

Answer 23

concentrated ammonia

Question 24

why do tertiary react faster then secondary and primary

Answer 24

Tertiary is the fastest / primary is
the slowest
The C-Br bond is weakest in
2-methylbromopropane / in the
tertiary (compound)
(This is because the) methyl
groups donate electrons and tertiary are electron rich and less electronegative carbon atoms

Question 25

what happens if halogalkane is reacted with OH- in aqueous conditions.

Answer 25

If it occurs in aqueous conditions, a nucleophilic substitution occurs, resulting in the formation of an alcohol. However, when it occurs in ethanol under reflux conditions, an alkene is produced in an elimination reaction.

Question 26

rate of hydrolysis on structure of bromoalkane

Answer 26

Tertiary is the fastest / primary is
the slowest

The C-Br bond is weakest in
in the tertiary (compound)

(This is because the) methyl
groups donate electrons

Question 27

Why is formation of alcohol better with alkali then water

Answer 27

Hydroxide ion /OH− is a stronger
nucleophile (than water) as full negative charge rather than partial so better electron pair donor

Question 28

why is ethanol added to test tube when doing the rate of hydrolysis of halogenoalkane

Answer 28

to dissolve the halogenoalkane

Question 29

why is apparatus left for 5mins before silver nitrate added

Answer 29

allow temperature to equilibrate and reach 50 degrees