Ochem question

[JustwantDDS]

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Answer is D. I don't know why I'm not getting it. What aldehyde did that come from? can someone draw it?

 

[Golfguy]

Answer is D. I don't know why I'm not getting it. What aldehyde did that come from? can someone draw it?

There is an H coming off of the C attached to the -OH group that's not drawn in. Tricky!

 

[wired202808]

There is an H coming off of the C attached to the -OH group that's not drawn in. Tricky!

Thats not why. Answer A has carbonyl... a hemi-acetal and acetals DO NOT have carbonyls. A and C cannot be the answer no matter what and should be eliminated right away. then the answer boils down to B and D and remember B is a acetal (it contains 2 -o- groups that are attached to something OTHER than a H) Therefore B is an acetal and D is a hemi-acetal. Hope this helps.

 

[Golfguy]

Thats not why. Answer A has carbonyl... a hemi-acetal and acetals DO NOT have carbonyls. A and C cannot be the answer no matter what and should be eliminated right away. then the answer boils down to B and D and remember B is a acetal (it contains 2 -o- groups that are attached to something OTHER than a H) Therefore B is an acetal and D is a hemi-acetal. Hope this helps.

The OP didn't understand where the aldehyde was, and I pointed it out. That allows one to easily see where the hemi-acetal originates....

 

[wired202808]

The OP didn't understand where the aldehyde was, and I pointed it out. That allows one to easily see where the hemi-acetal originates....

You dont need to look for an aldehyde or ketone in either a hemi acetal or acetal. All you look for are two -O- groups such as OH and OCH3 for a Hemi Acetal and a OCH3 and OCH3 for an Acetal. Thats the fastest way to get the problem right.

 

[JustwantDDS]

i still dont see what aldehyde it came from though...

I understand a hemiacetal has to O groups one OR, and one OH

 

[JustwantDDS]

assuming u have OCR2....

the carbonyl O becomes OH right, and then OR comes from the ROH u add

how can we have a cyclic aldehyde?
anyone get what i mean?

 

[JustwantDDS]

I wana know what R, R,, is from the asnwer

 

[wired202808]

assuming u have OCR2....

the carbonyl O becomes OH right, and then OR comes from the ROH u add

how can we have a cyclic aldehyde?
anyone get what i mean?

Ok so a carbonyl + 2 C2H5OH / H3O+; the carbonyl is broken and one more O is added to that carbon. Then both O's receive a C2H5 group. And thats that. Wikipedia it if you need to see the mechanism. Otherwise for future reference just look at the answers, you dont even have to worry about what came from where, they arent asking you to do the mechanistic conversion, theyre simply saying "Please pick the answer that has a C-O-C and then on the next carbon has an OH"

 

[JustwantDDS]

ok maybe im stupid or something but....
if u click on this

so this question says hemiACETAL right, so it had to have come from an aldehyde, how can we have D as the answer, thats cyclic. the O is part of a ring.

 

[mh0000]

ok maybe im stupid or something but....
if u click on this

so this question says hemiACETAL right, so it had to have come from an aldehyde, how can we have D as the answer, thats cyclic. the O is part of a ring.

google the reaction mechanism of the addition of an alcohol to an aldehyde. You will see that the reaction is reversible with the reverse reaction favored in concentrated acid. The acid will protonate the ether O to make it a better leaving group. The hydroxy O's lone pair will displace the leaving group. This breaks and opens the ring. Finally a base abstracts the H from the newly formed carbonyl to yield an aldehyde.

 

[wired202808]

ok maybe im stupid or something but....
if u click on this

so this question says hemiACETAL right, so it had to have come from an aldehyde, how can we have D as the answer, thats cyclic. the O is part of a ring.

you're not stupid... but your missing the POINT! They are NOT ASKING you to form the product... the answer is correct, because that is a Hemi Acetal. It is a glucose and one of the Oxygen's exists as an OH because thats the starting product. also dont stress over things that arent asked. I understand if they asked you to form the product; but your making wild assumptions w/o knowing the starting product.

 

[Barking Lamb]

This is really a naming question. Hemi-Acetal is half an acetal (That's what hemi means).

So what is an acetal? If you have a RO-C-OR this is an acetal.

Hemi-(half)-acetal is RO-C-OH. (Answer 'D' clearly has the hemiacetal.)

________________________________________________________________

Remember that hemiacetals and acetals come from the breaking apart of a carbonyl (C=O) group. Hemiacetals can be thought of as an intermediate product in many cases.

Carbonyl ----> Hemiacetal -------> Acetal

@DDS- You specifically asked how we make this cyclic?

HO-C-C-C-C-COH -------------> The molecule attacks itself and forms a cyclic ring.

 

[JustwantDDS]

This is really a naming question. Hemi-Acetal is half an acetal (That's what hemi means).

So what is an acetal? If you have a RO-C-OR this is an acetal.

Hemi-(half)-acetal is RO-C-OH. (Answer 'D' clearly has the hemiacetal.)

________________________________________________________________

Remember that hemiacetals and acetals come from the breaking apart of a carboxylic acid group. Hemiacetals can be thought of as an intermediate product in many cases.

Carbonyl (On the carboxylic acid attacked)----> Hemiacetal -------> Acetal

@DDS- You specifically asked how we make this cyclic?

HO-C-C-C-C-CO2H -------------> The molecule attacks itself and forms a cyclic ring.

thank you!! and everyone else!!