esters (1 Viewer)

[TheGreatest99.95]

i was going through a Q and it was about the Melting point and Boiling point of alkanoic acids, alcohols and esters.
I know that alkanoic acids can experience two hydrogen bonds and alcohols one. With esters they have dipole-dipole intermolecular forces and therefore giving them a lower BP and MP than alcohols and alkanoic acids. So my question is, why doesnt an ester experience H bonding? is it due to the large molecule size or something else


Cheers

 

[2xL]

Probably hazard a guess how the atoms of an ester don't share the electrons unequally like the O-H group of alcohols and carboxylic acids. In a hydroxyl group, due to very different electronegativities, the electrons are shared unequally giving rise to small charges and this makes gives rise to the polar nature of the OH group. Such an effect doesn't exist in esters.

EDIT : Went through wikipedia; has some complicated stuff on hydrogen bond acceptor and hydrogen bond donator...then checked syllabus and its not necessary to compare MP/BP of esters. Only Alkanoic Acid/Alkanol/Alkane.

 

[TheGreatest99.95]

Probably hazard a guess how the atoms of an ester don't share the electrons unequally like the O-H group of alcohols and carboxylic acids. In a hydroxyl group, due to very different electronegativities, the electrons are shared unequally giving rise to small charges and this makes gives rise to the polar nature of the OH group. Such an effect doesn't exist in esters.

EDIT : Went through wikipedia; has some complicated stuff on hydrogen bond acceptor and hydrogen bond donator...then checked syllabus and its not necessary to compare MP/BP of esters. Only Alkanoic Acid/Alkanol/Alkane.

I didnt think it was in the syllabus thanks for the help good luck with your trials

 

[madharris]

Probably hazard a guess how the atoms of an ester don't share the electrons unequally like the O-H group of alcohols and carboxylic acids. In a hydroxyl group, due to very different electronegativities, the electrons are shared unequally giving rise to small charges and this makes gives rise to the polar nature of the OH group. Such an effect doesn't exist in esters.

EDIT : Went through wikipedia; has some complicated stuff on hydrogen bond acceptor and hydrogen bond donator...then checked syllabus and its not necessary to compare MP/BP of esters. Only Alkanoic Acid/Alkanol/Alkane.

question 14 http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf

asks you to find the bp of an ester - i got it wrong originally because I didn't know about it D:

 

[barbernator]

question 14 http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf

asks you to find the bp of an ester - i got it wrong originally because I didn't know about it D:

you don't have to know how to calculate the BP in this answer, you just need to note that within the structure of the other 2 molecules, they each will be able to form hydrogen bonds, whereas the ester will not and hence its boiling point will be less, so the answer is A

 

[someth1ng]

1. Esters CAN participate in hydrogen bonding if there is a partially positive intermolecular hydrogen atom in its vicinity however the single oxygen atom in the ester capable of hydrogen bonding in near the middle of the ester making the ester more non-polar than polar.
2. Ester CANNOT form hydrogen bonds with other esters because the hydrogen atoms needs to form an intramolecular bond to an electronegative atom (eg oxygen in carboxylic acids) and form a intermolecular bond to an electronegative atom (NOF).

For number 2, you will probably ask, why?
If the hydrogen is connected to something less electronegative (eg carbon), the hydrogen will not become partially positive enough to form intermolecular bonds that are strong enough.

.'. Most of the bonds in esters are either dipole-dipole bonds or dispersion forces.