SP3 substitution

Question 1

what is the order of reactivity for Sn1 reactions

Answer 1

T>S>P>Me

Question 2

what is the rate law for Sn1

Answer 2

First order v=K[haloalkane]

Question 3

what is the best solvent for Sn1

Answer 3

Polar protic solvent eg water, ethanol, methanol etc

Question 4

Describe and explain the order of stability of the sp3 centres

Answer 4

Me<p></p>

Question 5

what is the stereochemical outcome of Sn1

Answer 5

Racemisation at reacting centre

Question 6

What is the relationship between TS and intermediate in Sn1

Answer 6

TS will resemble the structure of the cation intermediate

Question 7

The best solvent for Sn2

Answer 7

Polar aprotic solvent eg cyclohexane, toulene

Question 8

Describe and explain order of reactivity in Sn2

Answer 8

Me>p>s>t

Increased steric hindrance to backside attack

Question 9

rate law of Sn2?

Answer 9

v=K[R3CX][Nu-] no intermediate formed only TS

Question 10

what is the stereochemical outcome of Sn2

Answer 10

Complete inversion of reacting centre due to backside attack of Nu from C-X bond

Question 11

Explain Sn2 reaction in terms of orbitals

Answer 11

Filled orbital on Nucleophile attacks empty sigma* on C-X bond and to get best overlap will attack at a 180* angle.

Question 12

What is the worst group for Sn2

Answer 12

Neo-pentyl group (Tert group on beta carbon from X) H on Carbon-containing C-X bond screens backside approach more effectively than a-methyl groups

Question 13

What the rate of Sn2 including benzylic and allylic hydrocarbons, explain…

Answer 13

p < allylic < benzylic < alpha-carbonyl

Accelerated by pi conjugation in TS

Question 14

How can the inversion of Sn2 be demonstrated

Answer 14

Equal and opposite rotations of lights, eg reactant =+13 while product = -13

Question 15

why do Sn1 form a racemic mixture and what is evidence of this

Answer 15

Forms trigonal planar carbocation intermediate so equal chance of attack from above or below. can be proven to be racemic by zero value for rotation of product.

Question 16

why would incomplete racemisation occur in Sn1

Answer 16

When poor leaving group used as still electrostatically attracted to carbonation and may linger, hinders the approach of Nu from that side leading to slight preference.

Question 17

What is the effect of solvent polarity in Sn1

Answer 17

Increases polarity increases rate of reaction, due to stabilisation of partially negative charge by solvent and also the ability of the solvent to stabilise anion by H-bonds

Question 18

What is dielectric constant (e) Describe how higher e effects Sn1

Answer 18

measure of the ability of solvent to screen electrostatic attraction between opposite charges.
The higher e the more stable ions and the more the attractive forces are screened and easier it is to separate charges in TS.

Question 19

What is the effect of solvent polarity in Sn2

Answer 19

increases polarity decreases ROR, as anionic nu stabilised by solution more than TS

Question 20

example of best solvents for Sn2

Answer 20

Acetone, DMSO, DMF, HMPA

Question 21

What is a naked anion?

Answer 21

Anions weakly solvated in polar aprotic solvent, and have little cation-anion association

Question 22

What factors determine leaving group ability

Answer 22

Strength of C-X bond

Stability of X- , pKa decreases with higher stability

Question 23

How can OH be converted into a good leaving group

Answer 23

react ROH with TsCl and pyridine, Forming Tosylate ROTs which is a good LG

Question 24

Describe methods to turn alcohol to an alkyl halide

Answer 24

1) Use PBr3 or PI3 can be continued at the end as more active P-Br bonds
2) protonation with strong acid HCl HBr (NaBr + H2SO4)
But tert alcohols react more readily via Sn1