[THC]

I'm studying for an org.chem. test and I have a couple of questions. It's mostly about alkenes, alkynes and reaction mechanisms.

(1)
Q: "What alkynes would you start with to prepare the following ketones by a hydration reaction? a) Pentan-2-one b) Hexan-3-one"
A:
a) Pent-2-yne
b) Hex-2-yne and/or hex-3-yne.

According to the solution guide, the correct answer in b is hex-2-yne. Why can't hexan-3-one be formed from hex-3-yne?

(2)
What is the reaction mechanism for but-1,3-diyne and HCl? I imagine it's similar to the a 1,4-addition for but-1,3-diene, but how do I draw the intermediate? HC#C-C+=CH2 (# is a triple bond)?

(3)
Is HC#C-C+=CH2 stable? What is more stable, HC#C-C+=CH2 or HC#C-C+=CH-CH3?

(4)
Is prop-1,2-diene stable?

(5)
Functional groups like -COOH, -CHO, -COCH3 etc. on benzene rings are deactivators, while groups like -OH, -OCH3 on benzene are activating. All the groups contain O in various forms, so what's the difference?

That's all for now, I hope somebody will clarify these problems. Thanks.

[0000000]

I do not have time for all of them that so I will explane only one

(5)

Some groups can be activators or deactivators depending to two effects:
inductive attraction, this take place when the directly bonded atom to the ring have partial positive or negative charge. If it have positive charge then it will attract the electrons, therefore it will make the ring less attractive to electrofiles, and vise verse.
The second one is the resonance effect, it hapens when the directly bonded atom to the ring have lonely electrons pair and with resonance it can "donate them" to the bond with the ring and there will be a double bond, this atom will be positively charged and the ring will be negative charget, therefore it will be more attractive to cations. (there is exception with the hallogenes, but in your case is not important)

Now, you can see that in every deactivator, the directly bonded atom is C which is connected to O atom. You know that O atom is more electronegative and that so it will have partial negative charge, and the C atom will have partial positive charge- because of that it will attract the p electones from the ring making it less attractive to cations.

In your activator every O atom can donate electrons to the bond with resonance, making the ring more attractive

[azmanam]

Start here:

http://www.cem.msu.edu/~reusch/VirtTxtJml/addyne1.htm
http://www.cem.msu.edu/~reusch/VirtTxtJml/addene1.htm
http://www.cem.msu.edu/~reusch/VirtTxtJml/special1.htm
http://www.cem.msu.edu/~reusch/VirtTxtJml/benzrx1.htm

[Yggdrasil]

(1)
Q: "What alkynes would you start with to prepare the following ketones by a hydration reaction? a) Pentan-2-one b) Hexan-3-one"
A:
a) Pent-2-yne
b) Hex-2-yne and/or hex-3-yne.

According to the solution guide, the correct answer in b is hex-2-yne. Why can't hexan-3-one be formed from hex-3-yne?

hex-3-yne is the same molecule as hex-2-yne.

[edit: this is wrong, see sjb's post below]

[sjb]

hex-3-yne is the same molecule as hex-2-yne.

It is? Let's have a look...

HC#CCH₂CH₂CH₂CH₃ (hex-1-yne, or n-butylacetylene)

H₃CC#CCH₂CH₂CH₃ (hex-2-yne, or methylpropylacetylene)

H₃CCH₂C#CCH₂CH₃ (hex-3-yne, or diethylacetylene).

(Don't have access to a drawing package on this PC, # means triple bond)

Sorry Yggdrasil, I think you're wrong.

As to why not hex-3-yne, I don't know. There's no regioselectivity issues like there may be with the 2-yne (you could form hexan-2-one as well from that I think)

S

[macman104]

EDIT: Nm

[THC]

Thank you for your explanations so far. I still need explanations to 2, 3 and 4, so I would appreciate any help

[azmanam]

2) yes

http://www.chem.ucalgary.ca/courses/351/Carey/Ch09/ch9-8.html

3) what are the key differences to the two structures?  what effect do those differences have on carbocation stability?

4)  read this page again, noting the table

http://www.cem.msu.edu/~reusch/VirtTxtJml/special1.htm

[THC]

2) yes

http://www.chem.ucalgary.ca/courses/351/Carey/Ch09/ch9-8.html

3) what are the key differences to the two structures?  what effect do those differences have on carbocation stability?

4)  read this page again, noting the table

http://www.cem.msu.edu/~reusch/VirtTxtJml/special1.htm

Very good links! I just finished reading them now, thank you for your help - I have no further question

(for now)