SP3 substitution Flashcards
what is the order of reactivity for Sn1 reactions
T>S>P>Me
what is the rate law for Sn1
First order v=K[haloalkane]
what is the best solvent for Sn1
Polar protic solvent eg water, ethanol, methanol etc
Describe and explain the order of stability of the sp3 centres
Me<p></p>
what is the stereochemical outcome of Sn1
Racemisation at reacting centre
What is the relationship between TS and intermediate in Sn1
TS will resemble the structure of the cation intermediate
The best solvent for Sn2
Polar aprotic solvent eg cyclohexane, toulene
Describe and explain order of reactivity in Sn2
Me>p>s>t
Increased steric hindrance to backside attack
rate law of Sn2?
v=K[R3CX][Nu-] no intermediate formed only TS
what is the stereochemical outcome of Sn2
Complete inversion of reacting centre due to backside attack of Nu from C-X bond
Explain Sn2 reaction in terms of orbitals
Filled orbital on Nucleophile attacks empty sigma* on C-X bond and to get best overlap will attack at a 180* angle.
What is the worst group for Sn2
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
What the rate of Sn2 including benzylic and allylic hydrocarbons, explain…
p < allylic < benzylic < alpha-carbonyl
Accelerated by pi conjugation in TS
How can the inversion of Sn2 be demonstrated
Equal and opposite rotations of lights, eg reactant =+13 while product = -13
why do Sn1 form a racemic mixture and what is evidence of this
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.
why would incomplete racemisation occur in Sn1
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.
What is the effect of solvent polarity in Sn1
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
What is dielectric constant (e) Describe how higher e effects Sn1
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.
What is the effect of solvent polarity in Sn2
increases polarity decreases ROR, as anionic nu stabilised by solution more than TS
example of best solvents for Sn2
Acetone, DMSO, DMF, HMPA
What is a naked anion?
Anions weakly solvated in polar aprotic solvent, and have little cation-anion association
What factors determine leaving group ability
Strength of C-X bond
Stability of X- , pKa decreases with higher stability
How can OH be converted into a good leaving group
react ROH with TsCl and pyridine, Forming Tosylate ROTs which is a good LG
Describe methods to turn alcohol to an alkyl halide
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
