Good O-Chem Questions That Stumped Me!

[topdent1]

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1. Why do polar aprotic solvents increase the rates of Sn2 reactions?

2. Why is acetone the best solvent for the reaction between alkyl chloride and potassium iodide?

3. Consider the reaction coordinate diagram for an SN1 reaction, that is, for a reaction in which the transition state is more highly charged than the reactants. How does a change to a more polar solvent affect this reaction coordinate diagram?

a. Energy of reactants is raised, and transition state energy remains about the same.
b. Energy of reactants is lowered slightly, and energy of transition state structure is lowered significantly more.
c. Energy of reactants is raised, and energy of transition state structure is lowered significantly.
d. Energies of reactants and transition state are lowered by about the same amount.
e. Energies of reactants and transition state are raised by about the same amount.

(answer: b)

4. What is the best term to describe the rearrangement that causes the formation of 2-methyl-2-butanol from 2-bromo-3-methylbutane? (answer: 1,2-hydride shift)

Anyone want to take a shot at explaining these? Thanks.

 

[Orgodox]

1. Why do polar aprotic solvents increase the rates of Sn2 reactions?

Stick with me for a min it'll make sense but Im gana start with the SN1 and Polar protic and understanding that will help understand SN2 polar aprotic....

Remember that in SN1 we have a carbocation. This carbocation is stabalized by the polar protic solvent. On the other hand, polar Aprotic solvents are not able to help stabilize any carbocation formation that would occur under SN1 conditions therefor for an SN1 we want a polar protic solvent to increase the rate.
On the other hand, Polar Aprotic solvents do not solvate or encapsulate the nucleophiles so they are free to approach the organic substrate unencumbered. This means that the rate of an SN2 reaction will be increased when Polar Aprotic solvents are used. Non-polar solvents are also unable to be attracted to the incoming nucleophile so that has a rate enhancing effect on an SN2 reaction.

 

[Orgodox]

2. Why is acetone the best solvent for the reaction between alkyl chloride and potassium iodide?

Acetone is a great solvent because it can dissolve many things in it and its Polar making it great for SN2 reactions Aprotic (and its also cheap) but this would have to be inline with SN2 parameters so wouldn't hold true for a tertiary alkyl chloride

 

[topdent1]

Stick with me for a min it'll make sense but Im gana start with the SN1 and Polar protic and understanding that will help understand SN2 polar aprotic....

Remember that in SN1 we have a carbocation. This carbocation is stabalized by the polar protic solvent. On the other hand, polar Aprotic solvents are not able to help stabilize any carbocation formation that would occur under SN1 conditions therefor for an SN1 we want a polar protic solvent to increase the rate.
On the other hand, Polar Aprotic solvents do not solvate or encapsulate the nucleophiles so they are free to approach the organic substrate unencumbered. This means that the rate of an SN2 reaction will be increased when Polar Aprotic solvents are used. Non-polar solvents are also unable to be attracted to the incoming nucleophile so that has a rate enhancing effect on an SN2 reaction.

I understand the function of the aprotic/protic part. What gets me, is what function does the polar part play. You said that it helps draw the nucleophile to the electrophile in both sn1 and e1. Is that right? I guess that makes sense.

 

[Orgodox]

4. What is the best term to describe the rearrangement that causes the formation of 2-methyl-2-butanol from 2-bromo-3-methylbutane? (answer: 1,2-hydride shift)

Think about what is happening here, its sn1 so once the bromine leaves you have H3C-CH+CH(CH3)2
This carbocation is secondary, if an H shirts over you get a tertiary then it gets attached. So its called a hydride ****.

 

[Mstoothlady2012]

1. Why do polar aprotic solvents increase the rates of Sn2 reactions?

Polar protic solvents are the ones that have -OH or -NH groups. These groups have ability to form hydrogen bond with the nucleophile and cluster around it. Nucleophile is very imp for SN2 rxn. So if this happens it slows down the rxn b/c nucleophile is "busy" dealing with those molecules of the protic solvent. Therefore aprotic solvents are better. But even in aprotic solvents, polar aprotic solvent is preferred b/c they can interact with the salt that accompanies the nucleophile. Hence it leaves the nucleophile alone which can quickly react with SN2 while holding back the salt. Like in case of NaCl, polar aprotic will interact with Na+ while leaving Cl- alone. So Cl- can undergo SN2 rxn w/o any interruption. I hope this makes sense. This is how I understand it 🙂

2. Why is acetone the best solvent for the reaction between alkyl chloride and potassium iodide?

Once again same concept. Acetone is aprotice but still polar. It will react with K, holding it back and lets go of I, which can quickly react with alkyl chloride and undergo SN2 rxn.

3. Consider the reaction coordinate diagram for an SN1 reaction, that is, for a reaction in which the transition state is more highly charged than the reactants. How does a change to a more polar solvent affect this reaction coordinate diagram?

a. Energy of reactants is raised, and transition state energy remains about the same.
b. Energy of reactants is lowered slightly, and energy of transition state structure is lowered significantly more.
c. Energy of reactants is raised, and energy of transition state structure is lowered significantly.
d. Energies of reactants and transition state are lowered by about the same amount.
e. Energies of reactants and transition state are raised by about the same amount.

(answer: b)

In SN1 its all about the carbocation! The more stabilized the carbocation, better chances for SN1. SN1 "likes" polar solvents b/c they have dipole charge. We want "electron rich" molecules in these solvents which can react with positive charge of the carbocation. Hence protic solvents are prefered which have electron rich atoms like O, N. These molecule interact with carbocation and stabilize it. This lowers the transition-state energy, so SN1 undergoes faster.

4. What is the best term to describe the rearrangement that causes the formation of 2-methyl-2-butanol from 2-bromo-3-methylbutane? (answer: 1,2-hydride shift)

I think this one will undergo SN1. Intially we have a secondary carbocation, which can switch the hydrogen with the methyl group that is next to it. In this case we will form tertiary carbocation which is more favored.

Anyone want to take a shot at explaining these? Thanks.

Hope this helps!

 

[Orgodox]

What gets me, is what function does the polar part play.

For an SN2 reaction we have an incoming nucleophile which may be charged and We dont want it to be solvated and if we use a nonpolar solvent that wont occur. Non-polar solvents are also unable to be attracted to the incoming nucleophile so that has a rate enhancing effect on an SN2 reaction.

 

[Mstoothlady2012]

For an SN2 reaction we have an incoming nucleophile which may be charged and We dont want it to be solvated and if we use a nonpolar solvent that wont occur. Non-polar solvents are also unable to be attracted to the incoming nucleophile so that has a rate enhancing effect on an SN2 reaction.

well, actually SN2 prefers polar aprotic solvent not non-polar. Read my explanation for the reason why.

 

[Orgodox]

Yea youre right woops meant polar aprotic. Both like polar solvents
thanks mstooth

 

[TeamGuo]

kk we use mad amounts of acetone in the lab to clean everything.

Acetone rulesss 😀