Chemistry 3719 Chapter 4
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Classify the following molecules as 1o, 2o, or 3o alkyl halides:
a) isopropyl bromide
b) t-butyl chloride
c) cyclohexyl bromide
Why do 3o alkyl halides react much slower than 1o alkyl halides in SN2 reactions?
The electrophilic carbon is quite crowded in 3o substrates so the nucleophile is blocked from approaching
Why do solvents such as water and alcohols slow down SN2 reactions?
The solvent hydrogen bonds to nucleophiles and blocks their approach to the electrophile
What is hyperconjugation?
The donation of electron density to a radical or carbocation from adjacent sigma bonds
Why are carboxylate anions such poor nucleophiles?
The charge is delocalized over two oxygens thus stabilizing the anion
Rank the following in terms of their decreasing rates in SN2 reactions: n-Butyl bromide; t-butyl bromide; sec-butyl bromide
- 1 n-BuBr (fastest)
- 3 t-BuBr (slowest)
- 2 s-BuBr
Give the major product(s) from the following reaction: CH3CH2CH3 + Br2 + heat --->
CH3CH(Br)CH3 + HBr
Give the major product(s) from the following reaction: CH3CH2CH2OH + HBr ---->
CH3CH2CH2Br + H2O
What effect would adding excess nucleophile have on the rate of an SN2 reaction?
The reaction is bimolecular, depending upon concentration of reactants, so the reaction would go faster
Why are alkanes poor solvents in SN2 reactions?
Alkanes are non-polar, will not dissolve salts (nuclophiles), and will not help stabilize transition states
What type of solvents are best for SN2 reactions?
Polar aprotic (dissolve reagents but don't H-bond to nucleophiles)
What effect would adding excess nucleophile have on the rate of an SN1 reaction?
None; the rate of SN1 is independent of nucleophile concentration
Why is Br radical more selective than Cl radical?
Br radical is less reactive and the abstraction step is endothermic, which means the system can sense the type of radical being formed
Why are radical fluorinations and iodinations not practical?
Fluorinations are violently exothermic, iodinations are endothermic overall
Why are SN1 reactions faster for 3o substrates than 1o substrates?
SN1 involves carbocation formation; 3o carbocations are achievable since they are stabilized by hyperconjugation
Name the major organic product when methylcyclohexane undergoes radical bromination
Why is iodide anion more nucleophilic than fluoride anion?
Iodide is less electronegative than F so it is easier for it to share its charge with an electrophile
Which step is rate-determining in SN1 reactions?
Dissociation to the carbocation
Why is NH3 a better nucleophile than H2O?
N is less electronegative than O and so shares its lone pair more readily with electrophiles
Why are sulfonate esters better leaving groups than hydroxyl (OH)?
Sulfonate ester anions are resonance-stabilized and so are easier to form
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