[whyohme]

Hi, so this is a snap from my problems set solution.

In Part 4 - a ,  when ad HBr with ROOR to the 2-methylbutene, the Br is added onto the less substituted side. HOW COME?! Isnt it that H goes to the less sub side and halogen goes to the more sub side?

pls help

[Dan]

Isnt it that H goes to the less sub side and halogen goes to the more sub side?

What is the theory behind that statement? This would be expected if the reaction proceeded by an ionic mechanism.

The presence of peroxide initiates a different reaction mechanism involving radicals.

If you draw the mechanism and analyse the relative stability if the potential  intermediates you can start working towards an explanation.

[whyohme]

Isnt it that H goes to the less sub side and halogen goes to the more sub side?

What is the theory behind that statement? This would be expected if the reaction proceeded by an ionic mechanism.

The presence of peroxide initiates a different reaction mechanism involving radicals.

If you draw the mechanism and analyse the relative stability if the potential  intermediates you can start working towards an explanation.

the theory is Markovnikov's Rule. It states on my textbook that halogen adds to the more highly substituted side of the alkyne bond and hydrogen adds to the less highly substituted side .

[gritch]

the theory is Markovnikov's Rule. It states on my textbook that halogen adds to the more highly substituted side of the alkyne bond and hydrogen adds to the less highly substituted side .

As dan stated Markovnikov's Rule is observed because it proceeds through an ionic mechanism. The presence of the peroxide and the anti-Markovnikov product indicates a free radical mechanism instead. Free radicals aren't always covered in basic organic chemistry courses but here is a brief bit of extra reading if you're interested: http://www.chem.uky.edu/courses/che230/FL/chapter%2011.pdf

[Dan]

the theory is Markovnikov's Rule. It states on my textbook that halogen adds to the more highly substituted side of the alkyne bond and hydrogen adds to the less highly substituted side .

Markovnikov's rule is the name of the observation, but do you understand why and therefore when Markovnikov's rule holds?

"It is stated in the textbook" is not an explanation of why something happens. To understand the so called "peroxide effect", I think you need to understand the theoretical basis of the Markovnikov rule. The logic behind the explanation of the regioselectivity of the ionic and radical mechanisms is the same (relative stability of intermediates).

If you draw the mechanism and analyse the relative stability if the potential intermediates you can start working towards an explanation.

[whyohme]

the theory is Markovnikov's Rule. It states on my textbook that halogen adds to the more highly substituted side of the alkyne bond and hydrogen adds to the less highly substituted side .

Markovnikov's rule is the name of the observation, but do you understand why and therefore when Markovnikov's rule holds?

"It is stated in the textbook" is not an explanation of why something happens. To understand the so called "peroxide effect", I think you need to understand the theoretical basis of the Markovnikov rule. The logic behind the explanation of the regioselectivity of the ionic and radical mechanisms is the same (relative stability of intermediates).

If you draw the mechanism and analyse the relative stability if the potential intermediates you can start working towards an explanation.

ah ok thanks i will read over again