Aliphatic reaction mechanisms

Question 1

Define an electrophile

Answer 1

an electron deficient species and will react by accepting electrons In order to grain a filled valence shell
can either be positively charged species e.g. H+ or a neutral species e.g BF3

Question 2

Define a nucleophile

Answer 2

an electron rich species- has electrons available for donating to electron deficient centres
donates 2 electrons if it is either negatively charged or neutral but carrying a non bonding pair of electrons

OH-, H3N

Question 3

What is a leaving group and what characterises a good LG?

Answer 3

ion or neutral molecule that is displaced by a Nu
A good LG is one that forms a stable ion or neutral molecule after leaving the substrate.
Also related to basicity - if theres a strong base it is a poor LG, good NU and visa verse

Question 4

Features of an SN1 reaction

Answer 4

unimolecular
Rate = k[LG]
Nucleophilicity is not important as the carbocation is highly reactive
Nu can attack from either face of the planar species so can product a mixture of 2 stereoisomers (enantiomers) - its a racemic mixture as there is an equal probability of attack on each face.

Question 5

Features of SN2

Answer 5

Bimolecular
Rate = k[substrate][nucleophile]
Occurs via a penta-coordinate trigonal bypyramidal transition state to give a product in which the configuration of the carbon has been inverted.
Creates inversion - this is because electrostatic repulsion prevents Nu attack in the region of the LG so the Nu attacks from the opposite side to the LG = inversion.

The mechanism involves a high energy transition state in which the Nu and LG

Question 6

Features of E1

Answer 6

unimolecular, Rate = k [Substrate]
Once formed, carbocation can be attacked by the Nu but the Nu acts as a base, removing a proton from a position adjacent to the positive centre and original bonding electron are used to discharge a positive charge and make a double bond

Rotation about single bondsman occur which can lead to mixers of configurational isomers (E and Z) but generally E1 predominantly E alkenes.

Question 7

Features of E2

Answer 7

Bimolecular, Rate = k[base][substrate]
H-C and C-Y bond breaking could be concerted. 1 step process that passes through a single transition state

Has a good LG due to resonance stabilisation through the Pi framework

Stereospecific
anti-peri planar conformation

Question 8

why does E2 go through anti-peri planar?

Answer 8

1- elimination is occurring from the lower energy staggered conformer rather than the higher energy eclipsed conformer

2. the attacking base and departing leaving group are as far apart as possible in the transition state for anti-peri planar elimination

Question 9

What is stereospecific?

Answer 9

reaction that leads to production of a single isomer as a direct result of the mechanism of reaction and stereochemistry of starting material (i.e 1 isomer of SM = 1 isomer of product)
E2 and SN2

Question 10

What is stereoselective?

Answer 10

Gives rise to >1 predominant product because the pathway has a choice i.e. 1 isomer of SM = more than 1 isomer of product

E1 or SN1

Question 11

State 3 factors that can be altered to favour SN2 over SN1

Answer 11

1. solvent effects - to favour SN1 can transfer from a hydroxylic to a polar aprotic solvent e.g DMSO because they dissolve ionic Nu’s but don’t reduce the nucleophilicity by solvation - the Nu is needed
2. add a good Nu e.g OH- = the rate of SN2 is dependent upon the nature of the Nu. Whereas SN1 isn’t so by having a good Nu it will favour SN2.
3. Get a substrate that can form a primary carbocation - SN1 is more favoured at primary carbocations than SN1 which is favoured at tertiary carbocations.