jacksonmeg
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can ketones and aldehydes hydrogen bond with water or any other compound capable of hydrogen bonding?
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username913907
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(Original post by jacksonmeg)
can ketones and aldehydes hydrogen bond with water or any other compound capable of hydrogen bonding?
Guess the interaction between the lone pair electrons on the carbonyl O and the H from water would be strong enough to class as H bonding...

I open this to the floor...... Am I being dumb? (I mean the other degree educated chemists here)
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RagaZ
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The small aldehydes and ketones are freely soluble in water but solubility falls with chain length. For example, methanal, ethanal and propanone - the common small aldehydes and ketones - are miscible with water in all proportions.The reason for the solubility is that although aldehydes and ketones can't hydrogen bond with themselves, they can hydrogen bond with water molecules. One of the slightly positive hydrogen atoms in a water molecule can be sufficiently attracted to one of the lone pairs on the oxygen atom of an aldehyde or ketone for a hydrogen bond to be formed.

There will also, of course, be dispersion forces and dipole-dipole attractions between the aldehyde or ketone and the water molecules. Forming these attractions releases energy which helps to supply the energy needed to separate the water molecules and aldehyde or ketone molecules from each other before they can mix together.As chain lengths increase, the hydrocarbon "tails" of the molecules (all the hydrocarbon bits apart from the carbonyl group) start to get in the way. By forcing themselves between water molecules, they break the relatively strong hydrogen bonds between water molecules without replacing them by anything as good. This makes the process energetically less profitable, and so solubility decreases.

http://chemwiki.ucdavis.edu/?title=Organic_Chemistry/Aldehydes_and_Ketones/Properties_of_Aldehydes_%26_Keto nes/Properties_of_Aldehydes_and_Keto nes

it literally took me 30 seconds to google this.... -_-
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charco
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(Original post by JMaydom)
Guess the interaction between the lone pair electrons on the carbonyl O and the H from water would be strong enough to class as H bonding...

I open this to the floor...... Am I being dumb? (I mean the other degree educated chemists here)
Curiously enough I spent a few hours about a year ago researching the same question. I came across several papers referring to an aldehydic hydrogen bond (or something such)

For example:

http://scitation.aip.org/content/aip...1063/1.1676210

I came to the conclusion that there was a bonding effect, although somewhat weak in comparison to the accepted interactions.
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username913907
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(Original post by charco)
Curiously enough I spent a few hours about a year ago researching the same question. I came across several papers referring to an aldehydic hydrogen bond (or something such)

For example:

http://scitation.aip.org/content/aip...1063/1.1676210

I came to the conclusion that there was a bonding effect, although somewhat weak in comparison to the accepted interactions.
Wow.... thats some perseverance!
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jacksonmeg
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(Original post by charco)
Curiously enough I spent a few hours about a year ago researching the same question. I came across several papers referring to an aldehydic hydrogen bond (or something such)

For example:

http://scitation.aip.org/content/aip...1063/1.1676210

I came to the conclusion that there was a bonding effect, although somewhat weak in comparison to the accepted interactions.
I only ask because I don't understand how dimers form from carboxylic acids, how does the OH of one group hydrogen bond to the C=O of the other?
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charco
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(Original post by jacksonmeg)
I only ask because I don't understand how dimers form from carboxylic acids, how does the OH of one group hydrogen bond to the C=O of the other?
But this is a classic hydrogen bond.

The hydrogen of the -O-H group is rendered partially positive by the bonded electronegative oxygen.

One of the lone pairs on the carbonyl oxygen,C=O: is attracted to the partial positive hydrogen of the -OH group.

-C=O:--->H-O-
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jacksonmeg
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(Original post by charco)
But this is a classic hydrogen bond.

The hydrogen of the -O-H group is rendered partially positive by the bonded electronegative oxygen.

One of the lone pairs on the carbonyl oxygen,C=O: is attracted to the partial positive hydrogen of the -OH group.

-C=O:--->H-O-
so why can't aldehydes and ketones :'(

is it because the OH of the carboxyl group increases the polarity of the C=O bond? and aldehydes and ketones dont have this increased polarity?

Nvm, just read that they can
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jacksonmeg
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(Original post by charco)
But this is a classic hydrogen bond.

The hydrogen of the -O-H group is rendered partially positive by the bonded electronegative oxygen.

One of the lone pairs on the carbonyl oxygen,C=O: is attracted to the partial positive hydrogen of the -OH group.

-C=O:--->H-O-
Also, perhaps you could help me with conjugation?

It says
''Conjugated systems are flat because orbital overlap is more effect when the P orbitals are parallel''

This doesn't make sense to me :/
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charco
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(Original post by jacksonmeg)
so why can't aldehydes and ketones :'(

is it because the OH of the carboxyl group increases the polarity of the C=O bond? and aldehydes and ketones dont have this increased polarity?

Nvm, just read that they can
Aldehydes and ketones cannot form normal hydrogen bonds. Water can form hydrogen bonds with them but they cannot form hydrogen bonds to themselves. There are weak dipole-dipole interactions (also very weak hydrogen type bonds - but this is not included in A level discussions)

The reason for this is that the aldehydes and ketones do not have a hydrogen atom directly attached to an electronegative element, so they do not have very positive hydrogen atoms.
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charco
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(Original post by jacksonmeg)
Also, perhaps you could help me with conjugation?

It says
''Conjugated systems are flat because orbital overlap is more effect when the P orbitals are parallel''

This doesn't make sense to me :/
In order for 'p' orbitals to be parallel and overlap effectively the molecule must be planar. This means that the hybridisation of the carbons bonding in the carbon chain has to be sp2 so that a planar arrangement is produced.

You should know that sp2 hybridisation produces three degenerate orbitals that orient as far apart as possible. This is a trigonal planar arrangement. This leaves the remaining 'p' orbital at right angles to the plane.

Hence, whenever carbon bonds to three other atoms it does so via sp2 hybridisation and the resultant geometry is planar. In this case the 'p' orbitals are brought into parallel positions enabling overlap and conjugation.
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