Chemistry 3719 Chapter 8
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Which is the better nucleophile, iodide anion or bromide anion?
Iodide anion (larger); it is less electronegative and thus it is easier to share a lone pair with the electrophile
Why is water a poor solvent for SN2 reactions?
Polar protic so it H-bonds to the nucleophile; also will not solvate organic substrates well.
When (+)-2-butanol reacts with HBr under SN1 conditions, what is the optical rotation of the product(s).
Optical rotation will be 0 since a racemic mixture will be formed.
Which solvent is better for SN
2 reactions and why?
The first one, dimethylformamide (DMF) since it is polar aprotic. It will solvate the materials but not mask the nucleophile.
What is a racemic mixture?
An equal (50:50) mixture of 2 enantiomers
Which of the following molecules will undergo the fastest SN
2 reaction with cyanide anion?
a. since it is primary and unhindered
Why does sodium methoxide give mixtures of ethers and alkenes when it reacts with secondary alkyl halides?
Methoxide is a good base as well as a good nucleophile; E2 starts to compete with SN2 on crowded substrates
Why do carbocations rearrange?
To become more stable, either through the relief of strain (e.g. 4-membered to 5-membered rings), or the formation of a better cation (e.g. 2o to 3o)
Why is -OSO2CF3 such a good leaving group?
Not only is the negative charge stabilized by resonance, the electron-withdrawing CF3 group spreads the charge out further
Why is hydroxide a better nucleophile than water?
Hydroxide is a lot more reactive since the oxygen is only bonded to one H and has 3 lone pairs. In water the O is covalently bonded to two H atoms and therefore neutral and stable.
What happens to the rate of the following reaction when more nuclophile is added?
This is an SN2 reaction (i.e. the RDS is bimolecular) so the rate will increase
What is happening to cause the skeletal change in the following reaction?
The 2o carbocation that is formed undergoes a hydride shift to generate a more stable 3o carbocation
Why is hexane a poor solvent for SN1 and SN2 reactions?
Hexane is very nonpolar and will not solvate polar species such as salts (often the nucleophile source). It will also not be able to stabilize polar transition states (SN2) or intermediates (SN1).
What stabilizes a tertiary carbocation over a secondary carbocation?
Hyperconjugation, the donation of electron density from adjacent single bonds. There are more of these adjacent bonds in a tertiary carbocation.
What happens to the rate of an SN1 reaction when the concentration of nucleophile is increased?
The rate does not change since the nucleophilic species is not involved in the rate-determining step
Which of the two species below is the stronger nucleophile and why?
The alkoxide (on the right) is stronger since its charge is localized. The carboxylate's charge is spread out and thus more difficult to share.
Why is iodide anion a better leaving group than fluoride anion?
The charge on the larger iodide anion is more spread out, which in general stabilizes a negative charge.
What stereochemical change occurs during an SN2 reaction?
The carbon being attacked undergoes a stereochemical inversion
Why does the reaction between 2-bromobutane and NaOCH3 product a large amount of alkene and much less of the SN2 product?
This secondary bromide is somewhat crowded so the nucleophile/base (NaOCH3) attacks the more accessible beta proton(s) resulting in a large amount of E2 product
What is a solvolysis reaction?
This occurs when the solvent participates in a reaction, for example as a nucleophile in the SN1 reaction
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