Functional Groups 1: Alkyl Halides Flashcards
Why are alkyl halides good reagents?
Versatile
Polarity due to difference in electronegativity of the halogen and carbon atom
What is PVC?
Polyvinyl chloride (PVC)
Good for pipes because it remains rigid in contact with water
What are CFCs?
Chlorofluorocarbons
Refrigerants
Damages ozone layer
Hydrocarbons now used instead
What are 3 common ways of synthesising alkyl halides?
1) From alcohols using phosphorous tribromide
2) From alcohols using thionyl chloride
3) From alkenes
Describe how alkenes can be used to synthesise alkyl halides.
Electrophilic addition
E.g. with HBr or Br2 (to make dibromo)
Constitutional isomerism in major and minor products
Describe how thionyl chloride and alcohols can be used to synthesise alkyl halides.
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)
Describe how phosphorous tribromide and alcohols can be used to synthesise alkyl halides.
Driven by strong affinity of phosphorous for oxygen
Typically an SN2 mechanism
List the effectiveness of halide leaving groups from weakest to strongest.
Fluoride, chloride, bromide, iodide
Which halide is the strongest leaving group and why?
Iodide is the strongest leaving group
Because the C-I bond is the weakest
Since iodine is the least electronegative halogen
What is a nucleophile?
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
What is an electrophile?
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
Give examples of negatively charged nucleophiles.
Hydroxide
Bromide
Cyanide
Methane thiolate
Give examples of neutral nucleophiles.
Ammonia
Water
Dimethyl sulfide
Trimethylphosphine
Give examples of neutral electrophiles.
Carbonyls
Aluminium trichloride
Boron trifluoride
Give examples of positively charged electrophiles.
H+
NO2(+)
H3O(+)
Describe how nitriles can be synthesised.
Nucleophilic substitution reaction with cyanide
Nitriles can then be converted into primary amines using a reducing agent such as LiAlH4
What is retrosynthesis?
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
Describe how alkynes can be used to generate nucleophiles.
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
Describe how quaternary ammonium salts can react with alkyl halides.
Nucleophilic substitution reaction
Only works with tertiary amides
Give an example of how you can synthesise primary alkyl amines.
By using sodium azide
1) Form the alkyl azide using sodium azide
2) Reduce using LiAlH4
(Carbon chain length stays the same)
Describe the usefulness of sodium hydroxide in nucleophilic substitution reactions.
Not very useful
Not carried out often on small molecules
The alcohol is usually cheaper and usually more easily available than the halide
Describe how the Williamson ether synthesis is carried out.
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
Describe how you make esters using carboxylic acids.
Carboxylic acids react with alcohols to make esters
You can also use carboxylic acids as nucleophiles but reaction with alcohols works better
Describe the Finkelstein reaction.
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
