need help with a Q on OCR Chains, Rings and Spectroscopy 2002 Jan.

I'll link to the paper itself incase I haven't explained it correctly, but the general gist of the question is that glycine has a much higher melting point than ethanoic acid. Which I get, sure, zwitter ions, etc.

but one of the points on trhe mark scheme just doesn't make sense to me. the last mark on there says this:

"any comment on why hydroxyethanoic acid is lower e.g. H-bonding holds crystals together"

now, as far as I understood it, both of these molecules should form hydrogen bonds...so I really am having trouble. I thought that hydrogen bonding just happened when there was an electronegative element bonded to a hydrogen?

I drew an incredibly crude drawing of what I thought hydrogen bonding, but I have to warn you...I'm not overstating it at all when I use the words "incredibly crude". It's pretty shocking.

links to the paper and mark scheme:

http://www.thepaperbank.co.uk/getpaper.php?paper=papers%2FAS-A+Levels%2FChemistry%2FChains+Rings+and+Spectroscopy%2F2814_2002_jan_p.pdf - Mark Scheme
http://www.thepaperbank.co.uk/getpaper.php?paper=papers%2FAS-A+Levels%2FChemistry%2FChains+Rings+and+Spectroscopy%2F2814_2002_jan_w.pdf - The paper itself

picture is attached...I think...

hbondingwtf.JPG28.7KB

Reply 1

nadrouse

in glycine, the zwitterions are held together by relatively strong ionic interactions, whereas in hydroxyethanoic acid the moleculles are only held together by weaker hydgrogen bonds. hope this helps.

Reply 2

HawkAhriman

so there isn't any h-bonding in glycine?

Reply 3

nadrouse

if there is any, it'll be negligible compared with the ionic bonding between (-NH3)+ and (CO2)-

Reply 4

HawkAhriman

ah, okay...fair enough. that's what I originally understood of the zwitterion thing but the mark scheme threw me off completely.

thanks for the help.