Chemistry: Post your doubts here!

[a_wiserME!!]

how do I do these kind of questions?

 

[a_wiserME!!]

why is it that the acid part is not reduced to an aldehyde?

 

[nehaoscar]

View attachment 58848

how do I do these kind of questions?

The question says Oxygen is divalent i.e it must have 2 bonds each
Phosphorus is trivalent i.e it must have 3 bonds each
It's just trial and error to try and form a structure in which each phosphorus has 3 bonds and each oxygen has 2 bonds
Try placing them alternately and the joining the bonds to get what's needed

 

[nehaoscar]

Winter 2014 - P43 - Q8-c
Part i I got the same as ms

Can someone please explain how R has 4 absorptions
And how P and Q have 5

I got S as 2 because of the CH3 and CHO groups
P = CH3 , CH2 , CHO , CH3-CO and which other groups??
Similarly which ones for Q and R??

 

[nehaoscar]

How is the mechanism nucleophilic substitution?
The CH3-CH2 attaches to the NH removing HBr, and the CH2 has a positive delta... so wouldn't it be electrophilic...
Please explain why it's nucleophilic :/

 

[Catherine_1]

Winter 2014 - P43 - Q8-c
Part i I got the same as ms

Can someone please explain how R has 4 absorptions
And how P and Q have 5

I got S as 2 because of the CH3 and CHO groups
P = CH3 , CH2 , CHO , CH3-CO and which other groups??
Similarly which ones for Q and R??

I will explain for R and then I guess you can go about the others.

Okay, so for R the compound is CH3CH2(CH3)CH2CHO
Now for determining the absorption in nmr we have to check the environment on both sides left and right.
1.One would be R-CHO
2.Second would be R-CH2-CHO
3.Third one would be (CH3)2-CH2-R
4.Fourth one would be CH3-CH2-R(Now in this case you will see both the CH3(by being attached to a CH2 group) have the same environment)

Note:The carbon in bold and italics implies one environment.

Hope it helps.

 

[nehaoscar]

I will explain for R and then I guess you can go about the others.

Okay, so for R the compound is CH3CH2(CH3)CH2CHO
Now for determining the absorption in nmr we have to check the environment on both sides left and right.
1.One would be R-CHO
2.Second would be R-CH2-CHO
3.Third one would be (CH3)2-CH2-R
4.Fourth one would be CH3-CH2-R(Now in this case you will see both the CH3(by being attached to a CH2 group) have the same environment)

Note:The carbon in bold and italics implies one environment.

Hope it helps.

Thanks a lot! Could you please check these

Q = CH3CH2CH(CH3)CHO
1. R-CHO
2. R-CH-CHO
3. R-CH(CH3)-R'
4. R-CH2-CH3
5. R-CH3

P = CH3CH2CH2CH2CHO
1. R-CHO
2.R-CH2-CHO
3.R-CH2-CH3
4. R-CH3
5. I'm not sure of which one will be the 5th one since all the environments are covered right?? :/ or will it be R-CH2-CH2-R'?

S = (CH3)3C-CHO
1. R-CHO
2. R-CH3

 

[a_wiserME!!]

The question says Oxygen is divalent i.e it must have 2 bonds each
Phosphorus is trivalent i.e it must have 3 bonds each
It's just trial and error to try and form a structure in which each phosphorus has 3 bonds and each oxygen has 2 bonds
Try placing them alternately and the joining the bonds to get what's needed

ok yep thank you!

 

[Catherine_1]

Thanks a lot! Could you please check these

Q = CH3CH2CH(CH3)CHO
1. R-CHO
2. R-CH-CHO
3. R-CH(CH3)-R'
4. R-CH2-CH3
5. R-CH3

P = CH3CH2CH2CH2CHO
1. R-CHO
2.R-CH2-CHO
3.R-CH2-CH3
4. R-CH3
5. I'm not sure of which one will be the 5th one since all the environments are covered right?? :/ or will it be R-CH2-CH2-R'?

S = (CH3)3C-CHO
1. R-CHO
2. R-CH3

You're welcome!
Okay so you've mostly done it right , just a few things:
Q = CH3CH2CH(CH3)CHO
1. R-CHO
2. R-CH3-CHO(This one won't be CH carbon, rather CH3)
3. R-CH(CH3)-R'
4. R-CH2-CH(Not CH3)
5. R-CH3

P = CH3CH2CH2CH2CHO
1. R-CHO
2.R-CH2-CHO
3.R-CH2-CH3
4. R-CH3
5. I'm not sure of which one will be the 5th one since all the environments are covered right?? :/ or will it be R-CH2-CH2-R'?[Yeah you are right it will be R-CH2-CH2]

Now for this one, frankly speaking, I am myself perplexed when considering what is give in the marking scheme, since as far as my knowledge goes:

S = (CH3)3C-CHO
1. R-CHO would show one absorption
2. R-CH3 would also show one absorption

Moreover, I also feel that R-C-CHO should also give one absorption since that again reflects a different environment but I am really unsure why ms says 2 absorptions.:/ If anyone can elaborate this it would be great?

 

[nehaoscar]

You're welcome!
Okay so you've mostly done it right , just a few things:
Q = CH3CH2CH(CH3)CHO
1. R-CHO
2. R-CH3-CHO(This one won't be CH carbon, rather CH3)
3. R-CH(CH3)-R'
4. R-CH2-CH(Not CH3)
5. R-CH3

P = CH3CH2CH2CH2CHO
1. R-CHO
2.R-CH2-CHO
3.R-CH2-CH3
4. R-CH3
5. I'm not sure of which one will be the 5th one since all the environments are covered right?? :/ or will it be R-CH2-CH2-R'?[Yeah you are right it will be R-CH2-CH2]

Now for this one, frankly speaking, I am myself perplexed when considering what is give in the marking scheme, since as far as my knowledge goes:

S = (CH3)3C-CHO
1. R-CHO would show one absorption
2. R-CH3 would also show one absorption

Moreover, I also feel that R-C-CHO should also give one absorption since that again reflects a different environment but I am really unsure why ms says 2 absorptions.:/ If anyone can elaborate this it would be great?

Okay thanks!

Btw for this one:
"Q = CH3CH2CH(CH3)CHO
2. R-CH3-CHO(This one won't be CH carbon, rather CH3)"
How would it be CH3 cause technically the CH is attached to the CHO and the CH3 is not directly attached to CHO right?

And for S, i think it's only 2 because it's the hydrogen NMR and since that C has no H's bonded to it it won't show any absorption ...?

 

[Zoha Ali]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s13_qp_12.pdf

can anyone please explain question 19?

 

[Catherine_1]

Okay thanks!

Btw for this one:
"Q = CH3CH2CH(CH3)CHO
2. R-CH3-CHO(This one won't be CH carbon, rather CH3)"
How would it be CH3 cause technically the CH is attached to the CHO and the CH3 is not directly attached to CHO right?

And for S, i think it's only 2 because it's the hydrogen NMR and since that C has no H's bonded to it it won't show any absorption ...?

Yeah you are right that the CH is attached to CHO, I have just denoted it in that way though it isn't exactly appropriate , I should have shown it simply as R-CH3 since it is only attached to CH but I wrote the CHO part to indicate is the branched CH3 and not the first one, or lol it could just be R'-CH3.
So for Q = CH3CH2CH(CH3)CHO
1. R-CHO
2. R'-CH3
3. R-CH(CH3)-R'
4. R-CH2-CH(Not CH3)
5. R-CH3
Oh yeah lol, it makes sense it is hydrogen NMR.

 

[zeejay]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s13_qp_12.pdf

can anyone please explain question 19?

the answer is B ......lets first see A) since in a X2 molecules both the atoms bonded are the same so there is no such important role of electronegativity, they will share the electrons equally and so no partial charges will develop and there wont be any permanent dipole bonds......C) is also not the answer because it mention permanent dipole bonds.... D) is also not the answer because when a liquid converts into a gas the intermolecular forces break down, not the intra-molecular forces so the average bond enthalpy doesn't make a difference..... B) is the answer because obviously as we go down the group elements will have more electrons and thus their molecules will also have more electrons. The instantaneous dipole and induced dipole forces will be greater for bigger molecules and thus the boiling point will also be higher

 

[DESTROYER1198]

does anyone know when the oct/nov 2015 grade threshold will be available?
thanks

 

[nehaoscar]

does anyone know when the oct/nov 2015 grade threshold will be available?
thanks

It's on here:
http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_w15_gt.pdf

 

[nehaoscar]

How is the mechanism nucleophilic substitution?
The CH3-CH2 attaches to the NH removing HBr, and the CH2 has a positive delta... so wouldn't it be electrophilic...
Please explain why it's nucleophilic :/

 

[donewithtime80]

http://www.cie.org.uk/images/164950-2016-specimen-paper-4.pdf

Q 9b (ii) and (iii) please

 

[donewithtime80]

http://www.cie.org.uk/images/164950-2016-specimen-paper-4.pdf

Q 8, reaction III.... what is the reaction? I guessed it would be alkane to alkene and HCl is eliminated...

but the mark scheme says : NaOH, heat in ethanol
what's the reaction?

 

[donewithtime80]

The combustion of ethanol is an exothermic reaction. This reaction occurs spontaneously at low temperatures but does not occur at very high temperatures. Explain why.

Ans: As temperature increases T∆S is more negative or –T∆S increases
At high temperature T∆S is more negative than ∆H (so ∆G is positive)

please explain this.

 

[Catherine_1]

Hey,
Can anyone please help me in Q2 e) part Paper 5 summer 2007?

http://papers.xtremepapers.com/CIE/...d AS Level/Chemistry (9701)/9701_s07_qp_5.pdf

Please?
Thank you. :")