Synthesis Flashcards
alkene to dihaloalkane
halogenation, electrophilic addition.
Diatomic halogen is required.
alkene to haloalkane
halogenation, electrophilic addition.
HCl HBr HI etc required
haloalkane to alkene
elimination.
alcoholic KOH required.
alkane to alkene
cracking
catalyst required
alkene to alkane
hydrogenation, electrophilic addition.
H2/catalyst required
alkane to haloalkane
free radical substitution.
Diatomic halogen/ UV light required.
haloalkane to ether
nucleophilic substitution Sn1 or Sn2
Sodium or alcohol required
haloalkane to nitrile
nucleophilic substitution Sn1 or Sn2
KCN required
haloalkane to amine
nucleophilic substitution Sn1 or Sn2
NH3 required
haloalkane to alcohol
nucleophilic substitution Sn1 or Sn2
H2O/NaOH (aq) requried
nitrile to amine
reduction
amine to amide
condensation
carboxylic acid required
amide to amine
hydrolysis
NaOH then H+
amide to carboxylic acid
hydrolysis
carboxylic acid to amide
condensation
nitrile to carboxylic acid
acid hydrolysis
dilute HCl
carboxylic acid to carboxylate salt
base required
carboxylate salt to carboxylic acid
acid required
carboxylic acid to acid chloride
SOCl2, PCl3, PCl5 required
acid chloride to ester
condensation
alcohol required
carboxylic acid to ester
condensation
alcohol/conc. H2SO4 required
ester to carboxylic acid
hydrolysis
NaOH then H+
alcohol to ester
condensation
carboxylic acid/conc. H2SO4 required
ester to alcohol
hydrolysis
NaOH then H+
alcohol to alkoxide
Sodium required
alkoxide to ether
alcohol required
alcohol to alkene
elimination
Al2O3 or conc. H2SO4 required
alkene to alcohol
electrophilic addition, hydration
H2O/ dil. H2SO4 required
Primary alcohol to aldehyde
oxidation
Secondary alcohol to ketone
oxidation
Aldehyde to carboxylic acid
oxidation
carboxylic acid to aldehyde
reduction
aldehyde to primary alcohol
reduction
ketone to secondary alcohol
reduction
homolytic bond fission
this type of bond-breaking produces two free radicals. Homolytic fission involves each atom retaining the electron it was contributing to the covalent bond and normally occurs when non-polar bonds break.
heterolytic bond fission
heterolytic fission is where one of the atoms takes both electrons which were shared and the other atom keeps none of the bonding electrons. This type of fission occurs most often when a polar bond breaks.
carbocation
the positive carbon ion that is produced during heterolytic fission. The atom that the carbon is bonded to is a more electronegative element than the carbon, resulting in a negative ion and a carbocation.
carbanion
the negative carbon ion that is produced during heterolytic fission. the atom that the carbon is bonded to is a less electronegative element than the carbon, resulting in a positive ion and a carbanion.
electrophile
electrophiles are atoms or groups of atoms which are deficient in electrons and are attracted to groups that can donate electrons
carbocations eg R+
other cations eg H+
electron-deficient centres
nucleophile
nucleophiles are atoms or groups of atoms which are rich in electrons and are attracted to positively charged centres.
carbanions eg R-
other anions eg OH-, CN-
molecules with lone pairs eg. NH3
complete combustion
when an alkane burns completely in oxygen to produce carbon dioxide and water
eg.
CH4 + 2O2 -> CO2 + 2H2O
incomplete combustion
when an alkane burns incompletely in oxygen to produce carbon monoxide, carbon (soot) and water.
eg.
3CH4 + 4O2 -> 2CO + C + 6H2O
substitution of a halogen with an alkane
the halogenation of an alkane is an example of hemolytic fission. The bromination of methane is an example of a chain reaction involving free radicals.
initiation:
Br2 -> Br* + Br*
propagation:
Br* + CH4 -> CH3* + HBr
CH3* + Br2 -> CH3Br + Br*
termination:
CH3* + CH3* -> C2H6
Br* + CH3* -> CH3Br
the initiation step is slow and determines the overall rate of the reaction. This is an unsatisfactory way of making haloalkanes as a mixture of products is formed. These substitution reactions which happen with the saturated alkanes are slow, require energy in the form of UV light and give a mixture of products.
Preparation of alkenes - elimination
Alkenes can be prepared by the dehydation of alcohols or by the elimination of a hydrogen halide from a haloalkane.
Dehydration of an alcohol into an alkene
The dehydration can be brought by:
- passing alcohol vapour heated aluminium oxide catalyst
- reacting alcohol with an excess of concentrated H2SO4/H3PO4
eg
C4H9OH -> C4H8 + H2O
Base-induced elimination of hydrogen halides
alkenes can be produced in the lab from haloalkanes. this elimination reaction removes the hydrogen halide
eg, catalysed with alcoholic KOH
C2H5Br -> C2H4 + HBr
unless the halogen is on an end carbon, two products (in addition to the hydrogen halide) can be obtained.
Electrophilic addition reactions with alkenes
Alkenes are unsaturated and with a reactive C=C double bond they undergo addition reactions easily.
Hydrogenation,
the alkenes don’t react with hydrogen under normal conditions. However, in the presence of a finely divided nickel catalyst and temperatures of 200oC, alkenes react by addition to form an alkane.
C2H4 + H2 -> C2H6
Electrophilic addition,
alkenes react readily with halogens, forming dihaloalkanes
C2H4 + Br2 -> C2H4Br2
Addition of hydrogen halides,
alkenes react readily with hydrogen halides forming haloalkanes.
C2H4 + HBr -> C2H5Br
the addition of hydrogen halides to assymetrical alkenes like propane can result in two products.
eg.
propene + HBr -> 1-bromopropane + 2-bromopropane
the more likely product can be predicted using Markovnikov’s Rule.
Acid catalysed addition of water,
in the presence of an acid catalyst, alkenes can react with water to form alcohols
the addition of water to asymmetrical alkenes can result in two products, the most abundant product can be predicted with Markovkinov’s Rule
Nucleophilic substitution of haloalkanes
Nucleophilic substitution involves a nucleophilic attack on the carbon atom of the polar carbon-halogen bond. The bond is polar due to a difference in electronegativity of the carbon and the halogen.
When refluxed with an aqueous solution of an alkali, a haloalkane undergoes nucleophilic substitution to form an alcohol. It is attacked by a nucleophile, OH-.
When sodium is reacted with an alcohol, a sodium alkoxide is formed in a nucleophilic substitution mechanism.
2Na + 2C2H5OH -> 2Na+C2H5O- + H2
sodium ethoxide dissolved in alcoholic is a good nucleophile when reacted with a haloalkane the product is an ether.
The haloalkane is attacked by the nucleophilic alkoxide ion
e.g C2H5O-
When a haloalkane is reacted with an ethanolic cyanide, the product of the nucleophilic substitution is a nitrile.
Acid hydrolysis of the nitrile produces a carboxylic acid. This reaction is synthetically useful as it is a method of increasing the chain length by one carbon atom, which can be very useful in organic synthesis.
Elimination of haloalkanes
when a haloalkane is refluxed with alcoholic potassium hydroxide, the elimination of the hydrogen halide takes place producing an alkene.
Stability of carbocations
primary haloalkanes (least) < secondary haloalkanes < tertiary haloalkane (most)
Sn1
In nucleophilic substitution, there is 1 species in the rate-determining step.
Sn2
In nucleophilic substitution, there is 2 species in the rate-determining step.
Physical properties of alcohols
They are water-soluble, although this solubility decreases rapidly with increasing chain length.
The boiling point of alcohols is usually considerably higher than corresponding molecular mass alkanes - these two properties being due to hydrogen bonding between neighbouring alcohol molecules.
Polyhydric alcohols (diols and triols) are generally very soluble, high boiling point liquids or solids as a result of hydrogen bonding.
The carbon chain of the molecule is non-polar and water-repelling. As the length of the carbon chain of the alcohol increases the solubility of the alcohol decreases.
Synthesis of alcohols
Acid catalysed hydration of alkenes - electrophilic addition,
alcohols for industrial use are produced by the acid-catalysed hydration of alkenes. For example the catalytic hydration of propene yields as the main product propan-2ol as a result of the addition.
Substitution of haloalkanes - nucleophilic substitutions,
alcohols can be prepared by refluxing the appropriate haloalkanes with sodium hydroxide solution.
Reduction of carbonyl compounds using LiAlH4 - nucleophilic reduction
mild oxidation of a primary alcohol produces an aldehyde which can be further oxidised to a carboxylic acid using a more powerful oxidising agent.
aldehydes are reduced to primary alcohols, ketones are reduced to secondary alcohols.
Markovkinov’s Rule
When a hydrogen halide adds across an asymmetrical carbon-to-carbon double bond, the major product is formed by the hydrogen adding to the carbon atom of the double bond which already has the greatest number of hydrogen atoms attached to it.
Dehydration of alcohols to alkenes
Dehydration of an alcohol to form an alkene can be done in 2 ways,
i. alcohol vapour is passed over heated aluminium oxide
ii. alcohol can be heated with an excess of concentrated sulphuric acid.
alcohols reacting with sodium
forming a sodium alkoxide
alcohols reacting with carboxylic acids
alcohols both react with carboxylic acids to form esters in a condensation reaction. concentrated sulphuric acid is added to the reaction mixture to increase yield of the ester. It does this by shifting equilibrium to the right.
alcohols reacting with acid chlorides
with acid chlorides, the esterification is faster and more vigorous and synthetically more value.
ethers
ethers are organic compounds having the formula:
R-O-R’, where R and R’ are alkyl groups.
Low molecular mass ethers are low boiling point liquids as there is no hydrogen bonding between the ether molecules. the small ethers are slightly soluble in water.
they are very flammable and form explosive peroxides on storage.
synthesis of ethers
ethers can be prepared by the nucleophilic attack of an alkoxide ion on a haloalkane.
when sodium is reacted with an alcohol, a sodium alkoxide is formed. when sodium alkoxide is reacted with a haloalkane, the product is an ether.
preparation of carboxylic acids
carboxylic acids can be prepared by the oxidation of primary alcohols or by the oxidation of aldehydes.
refluxing a nitrile with a strong acid or base brings about the hydrolysis of the nitrile, forming a carboxylic acid.
hydrolysis of esters or amides.
properties of carboxylic acids
the first four carboxylic acids are water soluble indicating polarity and hydrogen bonding. with increasing chain length, solubility decreases rapidly.
hydrogen bonding between neighbouring molecules is such that they can exist when pure as hydrogen bonded dimers in the vapour, liquid or solid state.
the existence of these dimers explains the relatively high boiling points of the carboxylic acids. these dimers only exist with the pure acids and not in aqueous solutions.
small acids have strong pungent smells
they are weak acids. being able to delocalise electrons helps stabilises the structure and explain why ethanoic acid is acidic whereas ethanol is neutral.
reactions of carboxylic acids
reactions with MAZIT (magnesium, aluiminium, zinc, iron, tin) metals forming a salt and hydrogen gas.
neutralisation of bases forming a salt and water.
alcohols react with carboxylic acids to form esters. concentrated sulphuric acid is added to the reaction mixture to increase the yield by removing water formed in the esterfication.
carboxylic acids can be prepared by the oxidation of primary alcohols. these reactions can be reduced using lithium aluminium hydride dissolved in ether to primary alcohols.
when heated with ammonia or an amine, carboxylic acids form amides through a condensation reaction.
amines
amines are compounds related to ammonia (NH3) in which one, or more, of the hydrogen atoms in the ammonia molecule are replaced by the alkyl groups.
they can be primary, secondary or tertiary.
physical properties of amines
hydrogen bonding between the polar N-H groups in primary and secondary amines causes higher melting point and boiling point than comparable molecular mass hydrocarbons or tertiary amines, where no such N-H group occurs.
the short chain primary and secondary amines are water soluble due to hydrogen bonding between the polar N-H groups and the polar O-H bonds in water molecules.
tertiary amines are water soluble as the lone pair of electrons on the nitrogen can Hydrogen bond with the polar H of the water molecule.
as usual, solubility and volatility decreases markedly as the carbon chain length in the amine increases.
amines as bases
the lone pair of electrons on the nitrogen atom in amines mean that they can accept a proton from water molecules, producing hydroxide ions. therefore they are bases.
reactions of amines
due to their basis character amines react with mineral and carboxylic acids to form salths.
aromatics
compounds containing benzene or benzene type ring structures. they belong to a family known as the arenes.
benzene
C6H6
Benzene is saturated despite its double bonds. this is shown by it not decolourlising bromine.
x-ray analysis shows that the 6 carbon atoms in the ring form a regular hexagon with all the carbon to carbon bonds in the ring being of an intermediate length between a carbon to carbon single bond and double bond.
all carbon and hydrogen atoms lie in the same plane.
reactions of benzene
benzene rings undergo electrophilic substitution reactions with a number of reagents.
chlorination of benzene, chlorobenzene can be prepared by reacting benzene with chlorine in the presence of either an iron (iii) chloride catalyst or aluminium chloride catalyst. the catalyst is required to generate the Cl+ ion which can act as an electrophile
bromination of benzene occurs in a similar manner when bromine is reacted with benzene in the presence of either an iron (iii) bromide catalyst or aluminium bromide catalyst.
nitration of benzene,
in nitration, the electrophilic species is the nitronium ion (NO2+) which is formed by mixing together concentrated nitric acid and sulphuric acid. the nitronium ion attacks the electron rich benzene ring to give a nitrobenzenium ion and then nitrobenzene.
sulphonation of benzene, sulphonation involves substitution of the sulphonic acid grouo (-SO3H) for a hydrogen atom in the benzene ring. this can be brought about by heating benzene with a slight excess of concentrated sulphuric acid. the electrophile is the SO3 molecule, although neutral, a powerful electrophile.
alkylation of benzene,
alkylbenzenes can be formed by reacting benzene with a haloalkane with aluminium chloride acting as a catalyst.
