Synthesis Flashcards
homolytic fission
normally occurs between pure covalent bonds, each atom will have equal share of bonding electrons, forms free radicals
heterolytic fission
normally occurs between polar covalent bonds, unequal share of bonding electrons, forms ions
haloalkanes
substituted alkanes with one or more halogen atom instead of a hydrogen atom
why do haloalkanes undergo nucleophilic substitution reactions?
presence of slight positive charge on carbon atom and polar bond with halogen makes haloalkanes susceptible to nucleophilic attack
monohaloalkanes undergo nucleophilic substitution reactions with:
- alcoholic alkoxides to form ethers
- aqueous alkalis to form alcohols
- ethanloic potassium cyanide to form nitriles- end nitrile contains one more carbon than the original haloalkane
- base induced elimination reactions to form alkenes
carboxylic acid preparation
oxidation of primary alcohols or aldehydes by heating them with acidified potassium dichromate
hydrolysis of esters, nitriles or amides by heating in the presence of acid or alkali catalyst
reactions of carboxylic acid
- condensation reactions with alcohols to form esters (sulfuric acid catalyst)
- reduction with lithium aluminium hydride to form primary alcohols
- react with amino groups to form amide links
- formation of salts by reaction with metals or bases
amines
derived from ammonia where one or more of the hyrogens are replaced with alkyl groups
properties of amines
- hydrogen bonding in primary and secondary amines (N-H) so higher boiling points than tertiary amines
- all amines are soluble- can form hydrogen bonds with water
- weak bases forming alkaline solutions
reactions of amines
- react with hydrochloric acid, sulfuric acid and nitric acid to form salts
- react with carboxylic acids to form salts, amide are formed if these are heated losing water
reactions of alcohols
- react with reactive alkali metals to form alkoxides
- form alkenes in dehydration (elimination) reactions of alcohols
- form esters by reacting with carboxylic acids or acid chlorides
preparation of alcohols
heating haloalkanes under refluc with sodium/ potassium hydroxide by nuclophilic substitution
acid catalysed hydration of alkenes
aldehydes/ ketones reacting with lithium aluminium hydride (reduction)
properties of alcohols
- high boiling points due to hydogen bonding
- small alcohol molecules are miscible in water but larger are insoluble (as non polar end of molecules increases, solubility decreases)
properties of ethers
- lower B.Ps than alcohols because they don’t have hydrogen bonding
- can form hydrogen bonds with water so small molecules are solutble
- flammable and when exposed to air form peroxides can be explosive
- can be used as solvents- chemically inert, volatile
preparation of ethers
formed when monohaloalkanes react with alcoholic alkoxides
preparation of alkenes
- dehyration of alcohols using aluminium oxide, concentrated sulfuric acid or phosphoric acid
- base induced elimination of hydrogen halides from monohaloalkanes
markinokov’s rule
hydrogen is mainly added to the carbon in the alkene that is already bonded to the most hydrogens in an addition reaction of alkenes
alkenes undergo electrophilic reactions in
- halogenation (addition of halogens)
- hydrohalogenation- H already has a slighly positive charge
why do alkenes undergo electrophilic reactions?
there is high electron density at C=C which attracts electrophiles
SN1 reactions
substution reactions involving a nucleophile in which 1 species is involved in the rate determining step
involves heterolytic fission forming two ions, one being a carbocation
carbocations formed are intermediate
has 2 steps, first being RDS
SN2 reactions
substution reactions involving a nucleophile in which 2 species are involved in the rate determining step
has 1 step
two species involved form an unstable trasition state
inductive effect
alkyl groups are electron donating and push electrons to the positively charged carbocation carbon which increases stability, means tertiary haloalkane is more likely to SN1
steric effect
alkyl groups are bulkier than H atoms so it is more difficult for nucleophile to attach and form partial bond on central carbon- tertiary haloalkanes are less likely to SN2
simplest aromatic compound
benzene
molecular formula of benzene
C6H6
how do you know benzene does not have double bonds?
does not readily decolourise bromine water
shape of benzene
linear hexagonal
benzene molecular orbitals
- sp2 hybridised
- three filled sp2 hybrid orbitals form sigma bonds with 2 neighbouring C atoms and a H atom
- leaves an elctron occupying a p orbital on each carbon atom, each of these orbitals overlap side on with two p orbitals on each side which forms a pi molecular orbital
- six electrons in the pi orbital are delocalised and shared by all 6 carbon atoms
why does benzene undergo substitution reactions rather than addition reactions?
delocalised electrons give benzene unusual stability so addition reactions would disrupt the stability of the ring
what type of reactions does benzene undergo?
delocalised electrons attract electrophiles which can substitute/ replace H atom(s)= electrophillic subtituion reactions
electrophilic substitution reaction mechanism with benzene
- electrophile attacks one of the carbon atoms in the ring
- forms an intermediate with a postive charge that can be stabillised by delocalisation around the ring
- the intermediate ion then loses the H+ ion to regain aromatic character and restore the system
examples of electrophilic substitution reactions in benzne and other aromatic compunds
alkylation, nitration, sulfonation, halogenation
when one of the hydrogen groups has been substituted in a benzene ring it is known as
a phenyl group (C6H5)
