[OChem] Alcohol with HCl reaction Question

[premed21]

I have an O-Chem question. When reacting alcohols with HCl, what type of alcohol reacts the fastest and why? Is it tertiary, secondary, or primary? The reaction proceeds either by Sn1 or Sn2 right?

Sn2: The OH gets protonated and becomes a good leaving group. The Cl attacks from other side and you get inversion of config. This would be better for primary alcohol.

One of the questions says: Which type of alkyl halide reacts most readily with HCl? The answer was tertiary. I am confused. Any help is appreciated.

Thanks!

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[loveoforganic]

It depends. Tertiary alcohols will always react with acid by SN1 / E1 mechanism. Primary alcohols will generally react with acid by SN2 mechanism. Secondary alcohols can kinda go either way. The reason for this is varying carbocation stability - is it stable enough for water to leave on it's own without another nucleophile coming in to kick it out?

 

[premed21]

Right, but the question is: Which type of alkyl halide forms best from reaction of an alchol with concentrated HCl? choices are: primary, secondary, or tertiary.

The answer is tertiary, but why not primary? Both Sn1 and Sn2 can occur right?

 

[sv3]

Right, but the question is: Which type of alkyl halide forms best from reaction of an alchol with concentrated HCl? choices are: primary, secondary, or tertiary.

The answer is tertiary, but why not primary? Both Sn1 and Sn2 can occur right?

I'm not sure what your source is trying to say by stating tertiary. Whether its SN1 or SN2 depends on the level of subtitution on the Carbon of the alcohol. Knowing that, you can know if its SN1 or SN2. I get the feeling the question is asking "what produces a more stable product, SN1 or SN2?" And to be honest, I don't know if one is favored over the other - that's why both types exist i figure - for different situations.

 

[mkracker]

It is tertiary because it forms the most stable carbocation intermediate. The more stable the intermediate, the more likely, a reaction will occur. The stability of the tertiary carbocation also means that less energy i.e enthalpy is needed for it's activation energy and thus forms more readily.

 

[FSAP]

It is tertiary because it forms the most stable carbocation intermediate. The more stable the intermediate, the more likely, a reaction will occur. The stability of the tertiary carbocation also means that less energy i.e enthalpy is needed for it's activation energy and thus forms more readily.

It probably does not matter for the MCAT, but as a heads up, enthalpy is not the same as energy and if IIRC plays no role in kinetics as it is a thermodynamic property.

For the OP, you got acidic protic solvent. SN1 is favored. Hence, tertiary will react faster.

 

[sv3]

It probably does not matter for the MCAT, but as a heads up, enthalpy is not the same as energy and if IIRC plays no role in kinetics as it is a thermodynamic property.

For the OP, you got acidic protic solvent. SN1 is favored. Hence, tertiary will react faster.

So the HCl is the source of the nucleophile and also the solvent?
just making sure. thanks

PS (the OP words the question differently in their second post, asking which product is formed best, not which reacts best.......not sure if that changes what you said above)

 

[mkracker]

It probably does not matter for the MCAT, but as a heads up, enthalpy is not the same as energy and if IIRC plays no role in kinetics as it is a thermodynamic property.

For the OP, you got acidic protic solvent. SN1 is favored. Hence, tertiary will react faster.

Did this stuff a while ago and was quoting from memory. Went back to check and according to TPR, the more stable tertiary carbocation will lower the energy of the transition state. Enthalpy is not the same as energy, but when you deal with activation energy and transition states, you express energy in terms of enthalpy.

 

[loveoforganic]

It never mentioned the solvent. Could potentially be in aprotic solvent, using anhydrous HCl bubbled through. It sounds like they're assuming it's SN1 no matter what.

 

[sv3]

Oh ok. So you can't say since HCl is involved its a polar protic solvent?
In that case, back to my initial conclusion in that you can't tell if its SN1 or SN2

 

[Bildo]

The answer is tertiary because an SN1 reaction is favored when using acid and a relatively weak nucleophile. Tertiary stabilizes the cation the best.

The reason for this is that I think H20 is a much better leaving group than Cl- is a base. An OH2+ group wants to leave as water, a Cl- ion is pretty dece just hanging out in solution.

 

[FSAP]

Enthalpy is not the same as energy, but when you deal with activation energy and transition states, you express energy in terms of enthalpy.

You never express activation energy in terms of enthalpy. It's apples and oranges. One is a kinetic property while the other is thermodynamic. If you can show me an equation that expresses Ea in terms of delta H, I will stand corrected.

The answer is tertiary because an SN1 reaction is favored when using acid and a relatively weak nucleophile. Tertiary stabilizes the cation the best.

Seems like the best answer. If you really want to know, there are charts in Orgo books on when SN1 or SN2 is favored. Usually, when I see an acid in a question though, I am thinking SN1.

 

[mkracker]

You never express activation energy in terms of enthalpy. It's apples and oranges. One is a kinetic property while the other is thermodynamic. If you can show me an equation that expresses Ea in terms of delta H, I will stand corrected.

It is right there in TPR Cracking The MCAT CBT page 310.

Here's what TPR says:

In the case of an endergonic or an exergonic, the vertical distance between the energy of the reactants and the energy of the products is a measure of ΔGsys. For exothermic or endothermic reactions, the vertical distance between the energy of the reactants and the energy of the products is a measure of ΔHsys. Additionally, the vertical distance between the reactants and the highest vertical point on the curve is significant. In the case of a graph of free energy versus reaction coordinate, this vertical distance is a measure of the free energy of activation, or how much free energy is required to activate, or start, the reaction. In the case of a graph of enthalpy versus reaction coordinate, this vertical distance is a measure of the enthalpy of activation, or how much heat is required to activate the reaction. For the MCAT, enthalpy of activation is the activation energy, Ea.

 

[FSAP]

It is right there in TPR Cracking The MCAT CBT page 310.

I don't have that book. You are probably right then. I took PChem awhile ago though. The only way this makes sense if it's an isolated system and dH = q, so enthalpy of activation is just the heat required to trigger the reaction. At the end of the day, it's just a technicality. I would call it heat, but it is change in enthalpy as well. It's just that I am still not sure if you can really express Ea in terms of dH, but that's way beyond the MCAT scope.

 

[mkracker]

For the purposes of the MCAT, Ea can be expressed as ΔH

 

[sv3]

interesting debate but i've never had to make that association or seen it. I'm using TPR hyperlearning and never seen the two equated. OR at least I've never had to associate them while doing any problems. If anything I personally try to remember to keep those two seperate. I've seen that point noted in many books regading how a catalyst can change Ea but never effects delta H.

 

[sv3]

The answer is tertiary because an SN1 reaction is favored when using acid and a relatively weak nucleophile. Tertiary stabilizes the cation the best.

The reason for this is that I think H20 is a much better leaving group than Cl- is a base. An OH2+ group wants to leave as water, a Cl- ion is pretty dece just hanging out in solution.

Correct me if im wrong please but i think we finally have a reason for the answer now. Cl- is a weak and unusual nucleophile given its stability. and for SN2 you need a strong nucleophile (OH-). Thus its SN1 and carbocations come into play

 

[loveoforganic]

Not entirely true. Treatment of an alcohol with thionyl chloride proceeds by a mechanism by which the alcohol is converted into a fantastic leaving group, then chloride ion comes in, via an SN2 mechanism, to kick out that leaving group.

Here's a very good link, with good explanations.

http://www.cem.msu.edu/~reusch/VirtualText/alcohol1.htm

Scroll down to the 'Hydroxyl Group Substitution' section.

 

[starbaduk]

Correct me if im wrong please but i think we finally have a reason for the answer now. Cl- is a weak and unusual nucleophile given its stability. and for SN2 you need a strong nucleophile (OH-). Thus its SN1 and carbocations come into play

That sounds about right to me.

 

[EkramVahsedi86]

It is right there in TPR Cracking The MCAT CBT page 310.

Here's what TPR says:

In the case of an endergonic or an exergonic, the vertical distance between the energy of the reactants and the energy of the products is a measure of ΔGsys. For exothermic or endothermic reactions, the vertical distance between the energy of the reactants and the energy of the products is a measure of ΔHsys. Additionally, the vertical distance between the reactants and the highest vertical point on the curve is significant. In the case of a graph of free energy versus reaction coordinate, this vertical distance is a measure of the free energy of activation, or how much free energy is required to activate, or start, the reaction. In the case of a graph of enthalpy versus reaction coordinate, this vertical distance is a measure of the enthalpy of activation, or how much heat is required to activate the reaction. For the MCAT, enthalpy of activation is the activation energy, Ea.

I have the TPR book also. I believe the following statement is incorrect: "For exothermic or endothermic reactions, the vertical distance between the energy of the reactants and the energy of the products is a measure of ΔHsys."

The underlined word should say enthalpy. Energy depends on Gibbs free energy. Enthalpy depends on heat. The TPR itself wrongly defines enthalpy as energy by saying that "For the MCAT, enthalpy of activation is the activation energy" when it should have said "For the MCAT, and in this TPR book..." The TPR must remain consistent with itself, so other stuff will naturally be wrong that depends on enthalpy being unequal to gibbs free energy.

Entropy always increases and therefore an endothermic reaction can sometimes be energetically favorable. The TPR did a horrible job in the chapter of thermodynamics in my opinion. I was 10x more confused than the OP when I did that chapter. The TPR is only helpful in the sense that you have to go to other sources to learn something.

Enthalpy of activation is NOT the activation energy, but if the TPR says for the MCAT that it is, then I just hope it wasn't a way to trim the chapter, because thermodynamics is very, very contradictory; i.e. gibbs free energy disappears from an isolated system every time that heat leaves/enters a closed system (within the larger isolated system) in a cyclical process.