Debrief - 9746 Chemistry

last SG · 2007-11-05T19:38:26+00:00

I interpreted alkaline to be alkaline. That's it. So given the 2 choices, I chose the salt which is a weaker conjugate base.

teraexa SG

zzz. lost 5 marks because i thought carbonates were considered as carbonyls T_T

but other than that still manageable. a bit harder than i expected though.

akinos SG

ho ho ho!!! song bo liao... all the ratio question one, i dunno how to do sia.. lol anyhow put them. i did Qn 1,2 ,3 and 4. Qn 5 to me.... is an alien question bcoz i suck at Protein questions.. Laughing overall still abit hard la~

xxxshadowxxx SG

do we write the theoretical ratio of 6:4 for 1bromo n 2bromo or the realistic one whereby 2bromo is higher than 1bromo?

btw hi everyone my first time here!

teraexa SG

3:2 i think. that's what i wrote anyway. in actual fact, the stability of the secondary radical is outweighed by the increased chances of 1bromobutane being formed.

xxxshadowxxx SG

yup..3:2 is the original one..bt i wrote 3:7 which is the experimental one stated in my notes..nvm..i think mayb the answer will depends on ur explaination tt follows

plasticpiranha SG

hi guys, im new here too Very Happy
btw, i think the paper was horrible... Shocked
im counting on proteins actually

r u talking bout the 3 carbonyl compounds? any idea what are those 3?
i got propanone, propanal, methanal
but then i cant distinguish between methanal and propanal Crying or Very sad so i guess theyre wrong?

also whats the type of reaction when amide becomes N- Na+??

xxxshadowxxx SG

i use fehling's solution to distinguish between aldehyde n ketone..tollen's oso can

teraexa SG

the 3 correct ones should be methanal, ethanal and propanone.

to distinguish propanone, just use Tollens' as it won't give silver mirror.

then between methanal and ethanal, add aqueous alkali iodine. ethanal will give yellow ppt while methanal won't.

When amide becomes N- Na+, it is an acid-base/neutralisation reaction (your sodium salt).

plasticpiranha SG

oops i made a mistake in naming just now :p (editted the post)
how do you know which 3 carbonyls are formed?
and how come ethanal is formed? the no of C atom doesnt tally then?

tangjing SG

My friend say....its not ethanal.....its ethanoic acid...... and ethanoic acid has a carbonyl group as well....he gave a lot of explanation and I still don't understand at all....=/

This paper is like my school's standard I feel...if not harder....and I nearly failed for my school's prelim chem paper 3

wishboy SG

Originally posted by tangjing:
My friend say....its not ethanal.....its ethanoic acid...... and ethanoic acid has a carbonyl group as well....he gave a lot of explanation and I still don't understand at all....=/

This paper is like my school's standard I feel...if not harder....and I nearly failed for my school's prelim chem paper 3

he trying to scare u Laughing

th3m0ment SG

wiki says that tertiary radical > secondary radical > primary radical in terms of stability. so it is like 2-bromobutane is the major product rather than 1-bromobutane. if i did not interpret wrongly, that is it.

RaaRaa SG

Quite hard man..... Cambridge really transform into something new liao Laughing

teraexa SG

so in this case, how would you be able to approximate the ratio?

this is the a levels. let's not think too much extra-A-levels stuffs. keep it SAD (Simple And Direct).

teraexa SG

That reaction cannot form ethanoic acid.

http://www.chembook.co.uk/chap22.htm

Check for section "22.3.10. Other reactions of carboxylic acids and their salts."

"Calcium methanoate gives methanal on heating, whereas the calcium salts of other carboxylic acids give ketones. If calcium methanoate is heated with the calcium salt of another carboxylic acid, the products will be mixed eg calcium methanoate plus calcium ethanoate will give methanal, propanone, and also ethanal."

plasticpiranha SG

how are we supposed to know this? its nowhere in the syllabus i can think of...
cambridge is really trying to kill us... Rolling Eyes

teraexa SG

I guess they want us to learn the art of deduction.

xxxshadowxxx SG

i just give a ratio that is larger for 2-bromo..cos i tink there is not much stuff to explain this ratio if u write the theoretical 3:2..n it is indeed true that 2-bromo is the major pdt

teraexa SG

Er i derived the 3:2 from the fact that the possible C-H bonds that can be substituted with a Br atom.

There are 6 such places for 1-bromobutane and 4 such places for 2-bromobutane.

duperlicious SG

teraexa which jc u from??u seem so lihai..ahhh think the time restrain of the paper make it tough Sad