Section F: organic functional groups Flashcards
why do we study functional groups?
- functional groups have a characteristic chemical behaviour
- the same functional group in different molecules behaves in the same way
- several functional groups in one molecules usually behave independently of one another
- knowledge of functional groups helps understand the metabolism of molecules and simplifies our understanding of how biomolecules react
which molecules does electrophilic addition involve?
alkenes
which molecules are formed by the electrophilic addition of alkenes?
alcohols and alkyl halides
what is Markovnikov’s rule?
for the addition of hydrogen halides to alkenes, the H adds to the C with the most H atoms attached ie. the reaction proceeds in such a way as to involve the most stable carbocation
define ‘regioselectivity’
the preference of one direction of chemical bond making or breaking over all others, resulting in the formation of a single regioisomer
describe the biosynthesis of cholesterol
- squalene —> cholesterol
- cascade reaction
- multiple electrophilic additions occur
- reactions are linked so that the product of one is the starting material for the next ie. each electrophilic reaction generates a carbocation that acts as an electrophile in the subsequent reaction
- the active site of the enzyme bends squalene into a particular conformation
- this brings together parts of the molecule that will react with each other
which molecules are formed by an elimination reaction?`
alkenes
which reaction does the extension of glycogen involve?
nucleophilic substitution
what is the reaction for the extension of glycogen?
UDP-glucose + glycogen (n residues) —> UDP + glycogen (n+1 residues)
what is the function of UDP in the extension of glycogen?
- it tags glucose to signal to the enzyme
- it activates glucose to take part in the reaction
- it is a very good leaving group
why does the SN2 reaction favour primary over tertiary molecules?
there is no steric hindrance of the molecule with a primary molecule, which there would be with the bulky alkyl groups of a tertiary molecule
why does the SN1 reaction favour tertiary over primary molecules?
the alkyl groups of the tertiary molecule stabilise the carbocation intermediate by the inductive effect
describe the SN2 reaction
- bimolecular nucleophilic substitution
- one step
- both species involved in the rate-determining step ie. rate of reaction depends on both [Nu] and [L]
- attack from the back side
- in transition state, both Nu and L are partially bonded to the central carbon
- fastest when the substrate is primary and slowest when it is tertiary
- needs reasonably strong nucleophile
- sterospecific
- results in an inversion of configuration
- one pure enantiomer as a substrate gives one pure enantiomer as the product
describe the SN1 reaction
- unimolecular nucleophilic substitution
- two steps
- the rate of reaction depends only on [substrate] and not on [Nu]
- the bond between carbon and the leaving group is broken before the entry of the nucleophile
- fastest when the substrate is tertiary and slowest when it is primary
- nucleophile can be weaker
- results in racemisation in which a pure enantiomer gives a mix of both enantiomers
what is the effect of using a protic solvent on the rate of SN2 and SN1` reactions?
SN2 - the rate of reaction is decreased since the solvent would form a solvent cage around the nucleophile, reducing its nucleophilicity and preventing any reaction; alternatively, the protons could combine directly with the nucleophile
SN1 - the solvent helps to stabilise the leaving group and carbocation, increasing the rate of reaction
what is the effect of the solvent cage on both SN2 and SN1 reactions?
SN2 - solvent cage around the nucleophile reduces its nucleophilicity and, since nucleophile must be relatively strong, will decrease the rate of reaction
SN1 - the solvent cage would also decrease the nucleophilicity of the nucleophile, but the reaction would still be favourable overall:
- the intermediate carbocation is stabilised, lowering its energy on an energy diagram
- the nucleophile is also stabilised, increasing the height of the peak slightly
- but, since the intermediate carbocation energy level has been decreased, the absolute peak height is still lower
- therefore no effect on the kinetics of the reaction
give an example of an SN2 reaction
norepinephrine —> epinephrine (adrenaline)
in which SAM is activated by methylation
give an example of an SN1 reaction
geranyl diphosphate —> geraniol
in which OPP is an excellent leaving group
give an example of a polar protic and aprotic solvent
polar - H2O
aprotic - diethyl ether CH3CH2OCH2CH3
which two reactions are competing mechanisms?
elimination and substitution
which two reactions are ‘opposites’?
electrophilic addition and elimination
what is the difference between E2 and E1?
E2 - one step
E1 - two steps; removal of leaving group is the rate-determining step
what is Zaitser’s rule?
the carbon most likely to lose a hydrogen is the one with fewer hydrogens (due to hyperconjugation)
how is a primary alcohol oxidised?
primary alcohol —> aldehyde —> carboxylic acid
