Functional Groups 1: Alkyl Halides

Question 1

Why are alkyl halides good reagents?

Answer 1

Versatile
Polarity due to difference in electronegativity of the halogen and carbon atom

Question 2

What is PVC?

Answer 2

Polyvinyl chloride (PVC)
Good for pipes because it remains rigid in contact with water

Question 3

What are CFCs?

Answer 3

Chlorofluorocarbons
Refrigerants
Damages ozone layer
Hydrocarbons now used instead

Question 4

What are 3 common ways of synthesising alkyl halides?

Answer 4

1) From alcohols using phosphorous tribromide
2) From alcohols using thionyl chloride
3) From alkenes

Question 5

Describe how alkenes can be used to synthesise alkyl halides.

Answer 5

Electrophilic addition
E.g. with HBr or Br2 (to make dibromo)
Constitutional isomerism in major and minor products

Question 6

Describe how thionyl chloride and alcohols can be used to synthesise alkyl halides.

Answer 6

1) Lone pairs on oxygen from the alcohol react with sulfur on SOCl2, forming an intermediate
2) -OH group is converted into -OSOCl (better leaving group)
3) Chlorosulfate intermediate undergoes nucleophilic substitution (SN2 if primary or secondary, SN1 if tertiary)
4) Chloride from SOCl2 replaces the chlorosulfite group, producing alkyl halide (by-product is sulfur dioxide)

Question 7

Describe how phosphorous tribromide and alcohols can be used to synthesise alkyl halides.

Answer 7

Driven by strong affinity of phosphorous for oxygen
Typically an SN2 mechanism

Question 8

List the effectiveness of halide leaving groups from weakest to strongest.

Answer 8

Fluoride, chloride, bromide, iodide

Question 9

Which halide is the strongest leaving group and why?

Answer 9

Iodide is the strongest leaving group
Because the C-I bond is the weakest
Since iodine is the least electronegative halogen

Question 10

What is a nucleophile?

Answer 10

A negatively-charged or neutral species that is able to donate a lone pair of electrons in a high energy, filled orbital to another atom

Question 11

What is an electrophile?

Answer 11

A positively-charged or neutral species that is able to receive a lone pair of electrons from a nucleophile
It should have an empty atomic orbital or a low-energy anti-bonding orbital

Question 12

Give examples of negatively charged nucleophiles.

Answer 12

Hydroxide
Bromide
Cyanide
Methane thiolate

Question 13

Give examples of neutral nucleophiles.

Answer 13

Ammonia
Water
Dimethyl sulfide
Trimethylphosphine

Question 14

Give examples of neutral electrophiles.

Answer 14

Carbonyls
Aluminium trichloride
Boron trifluoride

Question 15

Give examples of positively charged electrophiles.

Answer 15

H+
NO2(+)
H3O(+)

Question 16

Describe how nitriles can be synthesised.

Answer 16

Nucleophilic substitution reaction with cyanide
Nitriles can then be converted into primary amines using a reducing agent such as LiAlH4

Question 17

What is retrosynthesis?

Answer 17

Working backwards from a target molecule to work out the starting compounds
Represented with a double forward arrow
Retrosynthetic analysis is used to prepare a synthesis

Question 18

Describe how alkynes can be used to generate nucleophiles.

Answer 18

Deprotonate an alkyne
E.g. with NH2(-), comes from sodium amide, which is made by carefully adding sodium metal to liquid ammonia at low temperatures
Sodium amide is a very strong base and alkynic protons are weakly acidic

Question 19

Describe how quaternary ammonium salts can react with alkyl halides.

Answer 19

Nucleophilic substitution reaction
Only works with tertiary amides

Question 20

Give an example of how you can synthesise primary alkyl amines.

Answer 20

By using sodium azide
1) Form the alkyl azide using sodium azide
2) Reduce using LiAlH4
(Carbon chain length stays the same)

Question 21

Describe the usefulness of sodium hydroxide in nucleophilic substitution reactions.

Answer 21

Not very useful
Not carried out often on small molecules
The alcohol is usually cheaper and usually more easily available than the halide

Question 22

Describe how the Williamson ether synthesis is carried out.

Answer 22

Much more useful than reaction with sodium hydroxide
1) Add sodium to alcohol to form Na+ and O- species
2) Nucleophile (O- species) then reacts with the alkyl halide to form the ether
*Works just as well with R-SH as the starting material - results in formation of thioethers

Question 23

Describe how you make esters using carboxylic acids.

Answer 23

Carboxylic acids react with alcohols to make esters
You can also use carboxylic acids as nucleophiles but reaction with alcohols works better

Question 24

Describe the Finkelstein reaction.

Answer 24

Useful for preparing alkyl iodides - gives best yield as iodide is the best leaving group
Iodide salts such as potassium iodide
Reaction driven forward by KCl and KBr