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Halogen Alkanes

After trying to teach myself this topic I'm now realising I know nothing, if anyone could help me out that would be amazing!

In aqueous alkali or ethanolic conditions, 2-bromo-2-methylbutane undergoes either substitution or elimination reactions to produce an alcohol or a mixture of two alkenes, respectively. Give the structures and names of these three compounds.

You must account for the formation of any products made via nucleophilic substitution by referring to the mechanisms of the reactions involved. (12 Marks)
In aqueous alkali solution, you will get substitution (OH- ions will act as the nucleophile), i.e. the C-Br bond is broken and replaced by the OH- nucleophile, giving you the C-OH bond, i.e. the alcohol. This is the only possible substitution. This will then give you 2-methylbutan(e)-2-ol, or 2-hydroxy-2-methylbutane (but the former is preferred).

For aqueous ethanolic solution, you will then get the elimination reaction. This is where you form your alkenes. You can only form alkenes on the adjacent carbons to the carbon with the halogen, i.e. the carbons next to the carbon with the halogen. The adjacent carbon MUST have a hydrogen that can be removed. This leaves three options, either the carbon to the left (the methyl group) or to the right (the ethyl group) or the bottom (i.e. the methyl group) of the carbon connected to the halogen. Two of them will give you the same product however. I'll attach the mechanisms as a pic in a second. Then you just need to name all of them with all the mechanisms.

I hope this all makes sense. If I have made a mistake, please anyone feel free to correct me.

20200419_104208.jpg
(edited 4 years ago)
Reply 2
Original post by inoubliable
In aqueous alkali solution, you will get substitution (OH- ions will act as the nucleophile), i.e. the C-Br bond is broken and replaced by the OH- nucleophile, giving you the C-OH bond, i.e. the alcohol. This is the only possible substitution. This will then give you 2-methylbutan(e)-2-ol, or 2-hydroxy-2-methylbutane (but the former is preferred).

For aqueous ethanolic solution, you will then get the elimination reaction. This is where you form your alkenes. You can only form alkenes on the adjacent carbons to the carbon with the halogen, i.e. the carbons next to the carbon with the halogen. The adjacent carbon MUST have a hydrogen that can be removed. This leaves three options, either the carbon to the left (the methyl group) or to the right (the ethyl group) or the bottom (i.e. the methyl group) of the carbon connected to the halogen. Two of them will give you the same product however. I'll attach the mechanisms as a pic in a second. Then you just need to name all of them with all the mechanisms.

I hope this all makes sense. If I have made a mistake, please anyone feel free to correct me.

20200419_104208.jpg

Thank you so much!!

Is there a mechanism for the production of 2-methylbutan-2-ol?
Original post by Amethyst190
Thank you so much!!

Is there a mechanism for the production of 2-methylbutan-2-ol?

Yesss - nucleophilic substitution. I'll explain it but if you do not understand it, then I'll draw it :smile:.

Basicaly the :OH- nuclephile will attack the carbon that's bonded to the halogen (MAKE SURE TO DRAW CURLY ARROWS AND NOT STRAIGHT ONES and also make sure it goes to the carbon and NOT the bond). Then the second curly arrow will be from the C-Br bond to the bromine, if that makes sense? If you want to draw it, post a pic and I'll tell you if it's right (and also make sure to draw the product because that's always worth 1 mark! I made the important notes capital because a lot of people lose marks there)
Reply 4
Original post by inoubliable
Yesss - nucleophilic substitution. I'll explain it but if you do not understand it, then I'll draw it :smile:.

Basicaly the :OH- nuclephile will attack the carbon that's bonded to the halogen (MAKE SURE TO DRAW CURLY ARROWS AND NOT STRAIGHT ONES and also make sure it goes to the carbon and NOT the bond). Then the second curly arrow will be from the C-Br bond to the bromine, if that makes sense? If you want to draw it, post a pic and I'll tell you if it's right (and also make sure to draw the product because that's always worth 1 mark! I made the important notes capital because a lot of people lose marks there)

chem c1.jpg
So kinda like that?
Original post by Amethyst190
chem c1.jpg
So kinda like that?

perfecto. just one thing though, make sure the curly line starts on the actual bond, i.e. there is no gap. some examiners are really particular and will deduct a mark if it's not connected to the bond. otherwise, good stuff!
Reply 6
Original post by inoubliable
perfecto. just one thing though, make sure the curly line starts on the actual bond, i.e. there is no gap. some examiners are really particular and will deduct a mark if it's not connected to the bond. otherwise, good stuff!

Okay thank you so much!!
Reply 7
Original post by Amethyst190
Okay thank you so much!!


it’s very small things like this which can lose you one mark in an exam question, just to tidy up your mechanism a bit more.

Make sure the negative charge is over the oxygen on the OH minus part of the mechanism, you’ve drawn in in between the O and the H
The lone pair should be two dots not two crosses.
Also you should draw H3C - and not CH3 - as the bond is coming from the carbon not the hydrogen on the left side.
(edited 4 years ago)
Reply 8
Also does anyone know anything about Maxwell–Boltzmann curves?

Draw, with labelled axes, a curve to represent the Maxwell–Boltzmann distribution of molecular energies in a gas. Label this curve T1. On the same axes, draw a second curve to represent the same sample of gas at a lower temperature. Label this curve T2. Use these curves to explain why a small decrease in temperature can lead to a large decrease in the rate of a reaction. (7 marks)
Reply 9
Original post by Amethyst190
Also does anyone know anything about Maxwell–Boltzmann curves?

Draw, with labelled axes, a curve to represent the Maxwell–Boltzmann distribution of molecular energies in a gas. Label this curve T1. On the same axes, draw a second curve to represent the same sample of gas at a lower temperature. Label this curve T2. Use these curves to explain why a small decrease in temperature can lead to a large decrease in the rate of a reaction. (7 marks)


Yes draw the regular sketch of the curve perhaps at 298K. Then think about at a lower temperature where the peak of the curve would be and how the height of the curve would change. Less molecules of gas have an internal temperature the same or greater than the activation energy at a lower given temperature.

Changing the temperature of a reaction affects the speed at which the particles collide, as well as the number of particles that have an internal energy equal to or greater than the activation energy. This has a twofold effect on the reaction rate, as increasing the temperature increases the frequency of collisions, as well as making collisions more successful. So think about the opposite effect of this when decreasing the temperature and the impact of the reaction rate.
(edited 4 years ago)

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