[orgoclear]

The two questions are attached below

For Q1. My views.. approx equal because Hyperconjugation favours I (it has 5 H while II has 4) and inductive effect favours II (I of two ethyl>effect of 1methyl and 1 propyl)

Does symmetry have some effect here?

Q2. Out of clues

[stewie griffin]

For question 1, don't forget that C-C bonds can also do hyperconjugation... it doesn't have to be just a C-H bond. So compound II has hyperconjugation with  4 C-H bonds and 2 C-C bonds. Compound 1 has hyperconjugation with 5 C-H bonds and 1 C-C bond.
Now, I really can't remember if it's better (meaning more stabilization) to have hyperconjugation with a C-C or a C-H bond, but maybe you may have just covered this in class?

[nj_bartel]

Don't forget inductive effects.

[orgoclear]

For question 1, don't forget that C-C bonds can also do hyperconjugation... it doesn't have to be just a C-H bond. So compound II has hyperconjugation with  4 C-H bonds and 2 C-C bonds. Compound 1 has hyperconjugation with 5 C-H bonds and 1 C-C bond.
Now, I really can't remember if it's better (meaning more stabilization) to have hyperconjugation with a C-C or a C-H bond, but maybe you may have just covered this in class?

The book I use (Carey) also mentions the same. But our professor said that only C-H bonds are effective in hyperconjugation

Don't forget inductive effects.

Yeah I have mentioned that. Due to inductive effect. II is more stable (two ethyl) and due to hyperconjugation I is more stable.

How do I weigh these factors?

[nj_bartel]

I have this idea in my head that inductive effects play a much more significant role than hyperconjugation, but honestly, I don't remember where I learned that nor if I actually learned it!

[orgoclear]

I have this idea in my head that inductive effects play a much more significant role than hyperconjugation, but honestly, I don't remember where I learned that nor if I actually learned it!

Hmm Carey states that hyperconjugation being a no-bond resonance is a bit more stabilising than inductive effects. (but it says that is the case when the no. of hypers exceeds atleast 2)

But is hyper. of C-C more than hyper of C-H ?

[orgopete]

Perhaps this is the type of question that one try to decide what experiment could answer this question the best. I can understand that the answer would be ambiguous. For example, while II may have more hyperconjugation, the product of an elimination from this carbocation does not involve their contribution to product formation to as great an extent as from the CH2 (more inductively donating). However, alkene stability would be biased toward the more substituted alkene so that may not be the correct conclusion.

Perhaps the rates of solvolysis of 2- and 3-bromopentanes would give a better clue? Can anyone provide those react rates?