Chemistry: Post your doubts here!

[AbbbbY]

Yeah but thanks

Heres what hyou need to know for Benzene halogenation.

http://i.imgur.com/QOdkC8g.jpg

Almost all benzene reactions are shown like this. Very simple really.

 

[asma tareen]

I havent studied any benzene reactions :O

 

[AbbbbY]

I havent studied any benzene reactions :O

You must be giving AS only then.

You said Bromination so I just wrote both the cases AS and A2.

 

[asma tareen]

You must be giving AS only then.

You said Bromination so I just wrote both the cases AS and A2.

Ohh maybe thanks but m In AS

 

[Suchal Riaz]

Ohh maybe thanks but m In AS

http://www.chemguide.co.uk/mechanisms/eladd/symbr2.html#top

 

[Faaiz Haque]

MCQ QUESTIONS PLEASE HELP

Q6,17,20,22,24,25,26,27,28,29,38
With explanations, if you can help me with even one question, ty

http://papers.xtremepapers.com/CIE/...d AS Level/Chemistry (9701)/9701_s09_qp_1.pdf

 

[AbbbbY]

MCQ QUESTIONS PLEASE HELP

Q6,17,20,22,24,25,26,27,28,29,38
With explanations, if you can help me with even one question, ty

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s09_qp_1.pdf

6:
I'd have picked D.

It's between A and D for me. I'll take D because you're getting rid of the double bond so 120 -> significantly lesser.

In A, and Alcohol will be formed but I'm not sure what the bond angles are. Should be lesser on one of the bonds due to the O lone pairs. Still I'd go for D because of the double bond -> single bond.
Edit: I just figured, it's the central carbon atom bond that we need to see not the O-H bond so A is out. A angles are all 109.5, the ones that matter.
17:
D

A- Happening.
B- NH3 -> NH4
C- (NH4)+ and (SO4)2-
D- You're just left with D. If you're unsure and want to waste more time, SO3 s = +6 final S = (-2+S-8)=0 so S= +6 so no redox.

22-
Free Radical Substitution
D

You're subtituting the H with Cl. Remember the very first reaction? FRSR of Alkane? That's what is happening. Don't let the acid part fool you it's the alkane part that's reacting here.

24-
D
Nucleophilic Substitution so both are nucleophiles

25-
B
A gives you propene
B gives you 2 bromo propane
(Remember, Halogenoalkanes, if theres a reaction that's not under ethanolic condition, it'll form an alcohol)
C gives you propene
D Again gives you propene (dehydration of the alcohol)


26:
Oxidation of a primary alcohol can give you Aldehydes and Carboxylic Acids.

27:
B
Ethanol reacting with Butanoic Acid so Ethyl Butanoate

28:
Will get back to you tomorrow.

29:
D

I can't explain this in words. Just draw it out and see where the double bonds would go. Adjacent Carbons in this case. let me know if you dont get it and I'll draw them.

 

[Namehere]

MCQ QUESTIONS PLEASE HELP

Q6,17,20,22,24,25,26,27,28,29,38
With explanations, if you can help me with even one question, ty

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s09_qp_1.pdf

Q20:

B- 2^3 = 6. If you know about statistics this is a bit easier. There are 2 possibilites around each double bond, either cis or trans. You have 3 double bonds in which this cis-trans combination can occur, hence it is 2 x 2 x 2 aka 2^3 = 6 (2 combinations for first double bond, 2 for second, 2 for third.)

Q38: B - 1 & 2 are correct. If you look at the beginning of the molecule from the left you can see that KMnO4 will break the double bond, and since the C attached to the double bond is a tertiary carbon atom (since its attached to 3 carbon atoms), it will yield, amongst other products, a ketone - in this case propanone. 2 is also correct because halogens can add across double bonds via the electrophilic addition mechanism.

Hope it helps.

 

[Namehere]

Abbbbby... I have researched a bit for Q28, and this is what I found: http://en.wikipedia.org/wiki/File:Carvone_oxidation.png

Which I don´t get, plus, that product has 8 carbons and the question said X has 9 carbons. If you do understand please explain to me.

With regards to your answer, I dont get the cleavage you did to the double bond in the cyclic compound, the part where you opened it - I get the aldehyde part of where you opened it, but what about the other carbon atom in the double bond (the one next to the CH3 branch), its a tertiary carbon atom, shouldn´t you get a ketone there?

 

[AbbbbY]

Abbbbby... I have researched a bit for Q28, and this is what I found: http://en.wikipedia.org/wiki/File:Carvone_oxidation.png

Which I don´t get, plus, that product has 8 carbons and the question said X has 9 carbons. If you do understand please explain to me.

With regards to your answer, I dont get the cleavage you did to the double bond in the cyclic compound, the part where you opened it - I get the aldehyde part of where you opened it, but what about the other carbon atom in the double bond (the one next to the CH3 branch), its a tertiary carbon atom, shouldn´t you get a ketone there?

Abby. Two B's.

__

Interesting. Well it's 2 am and my mind's numb. I'll look into it again tomorrow. Removing the answer for now to avoid confusion among the masses.

 

[zackle09]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w10_qp_42.pdf

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w10_ms_42.pdf

can someone please tell me how to do part 3 of question 3bi

 

[Young Stunner]

Can someone tell me how is this 1 & 2

 

[Faaiz Haque]

6:
I'd have picked D.

It's between A and D for me. I'll take D because you're getting rid of the double bond so 120 -> significantly lesser.

In A, and Alcohol will be formed but I'm not sure what the bond angles are. Should be lesser on one of the bonds due to the O lone pairs. Still I'd go for D because of the double bond -> single bond.
Edit: I just figured, it's the central carbon atom bond that we need to see not the O-H bond so A is out. A angles are all 109.5, the ones that matter.
17:
D

A- Happening.
B- NH3 -> NH4
C- (NH4)+ and (SO4)2-
D- You're just left with D. If you're unsure and want to waste more time, SO3 s = +6 final S = (-2+S-8)=0 so S= +6 so no redox.

22-
Free Radical Substitution
D

You're subtituting the H with Cl. Remember the very first reaction? FRSR of Alkane? That's what is happening. Don't let the acid part fool you it's the alkane part that's reacting here.

24-
D
Nucleophilic Substitution so both are nucleophiles

25-
B
A gives you propene
B gives you 2 bromo propane
(Remember, Halogenoalkanes, if theres a reaction that's not under ethanolic condition, it'll form an alcohol)
C gives you propene
D Again gives you propene (dehydration of the alcohol)


26:
Oxidation of a primary alcohol can give you Aldehydes and Carboxylic Acids.

27:
B
Ethanol reacting with Butanoic Acid so Ethyl Butanoate

28:
Will get back to you tomorrow.

29:
D

I can't explain this in words. Just draw it out and see where the double bonds would go. Adjacent Carbons
in this case. let me know if you dont get it and I'll draw them.

Thanks. I think i'll be able to do 29. And for 24, how do we know it's a Nucleophilic Reaction?

 

[Faaiz Haque]

Q20:

B- 2^3 = 6. If you know about statistics this is a bit easier. There are 2 possibilites around each double bond, either cis or trans. You have 3 double bonds in which this cis-trans combination can occur, hence it is 2 x 2 x 2 aka 2^3 = 6 (2 combinations for first double bond, 2 for second, 2 for third.)

Q38: B - 1 & 2 are correct. If you look at the beginning of the molecule from the left you can see that KMnO4 will break the double bond, and since the C attached to the double bond is a tertiary carbon atom (since its attached to 3 carbon atoms), it will yield, amongst other products, a ketone - in this case propanone. 2 is also correct because halogens can add across double bonds via the electrophilic addition mechanism.

Hope it helps.

Ty I understand 20
For 38 I get why #2 is correct but #1?. Can you explain it further? Why will KMnO4 break the double bond

 

[Abdel Moniem]

Thank you so much

c(iii) th gas in (ii) is CO2 and if you see total of gas formed is 40 but only 30 remains when the tottal gas is shaken with KOH(aq) se where did the 10 go it reacted with Koh(aq) and the gas is CO2 so 10cm^3
iv) they gave 50 O2 whcih gave us total of 40 cm^3 of gases of which 10 is CO2 so we have 30 left This 30 is O2 which is left from 50 so 50-30=20cm^3 got it?
d) the ms explains it quite well though but here we will use the volume ratio from balancing in b the ration between A used is 10 and CO2 produced is 10 so 1 cm^3 of A give 1 cm^3 of Co2 x=1
and 10 cm^3 of A reacts with 20 cm^3 of O2 so 1cm^3 of A reacts 2 cm^3 of O2
x=1
2=x+(y/4)
2=1+(y/4)
y=4
CxHy will be CH4

 

[Abdel Moniem]

Cyanohydrins can be made from carbonyl compounds by generating CN–ions from HCN in the presence of a weak base.

In a similar reaction, –CH2CO2CH3 ions are generated from CH3CO2CH3 by strong bases. Which compound can be made from an aldehyde and CH3CO2CH3 in the presence of a strong base?
A CH3CH(OH)CO2CH3
B CH3CO2CH2CH(OH)CH3
C CH3CH2CH(OH)CH2CO2CH3
D (CH3)2C(OH)CH2CO2CH3

 

[Namehere]

Ok!

Ty I understand 20
For 38 I get why #2 is correct but #1?. Can you explain it further? Why will KMnO4 break the double bond

It´s something you have to know. Acidified KMnO4 is such a strong oxidising agent that it will oxidise and break the double bond to produce CO2 + H2O if the cleavage occurs at a primary carbon atom; an aldehyde if the cleavage occurs at a secondary carbon atom or a ketone if the cleavage occurs at a tertiary carbon atom. Do note however, acidified potassium dichromate is only used as an oxidising agent and will not break double bonds since it is not a strong enough oxidisng agent (just in case, for future questions...)

Hope you understand.

 

[AbbbbY]

Q20:

B- 2^3 = 6. If you know about statistics this is a bit easier. There are 2 possibilites around each double bond, either cis or trans. You have 3 double bonds in which this cis-trans combination can occur, hence it is 2 x 2 x 2 aka 2^3 = 6 (2 combinations for first double bond, 2 for second, 2 for third.)

Ooo I missed this question.

Right method, wrong answer. 2^3 = 8, not 6

The answer should be C.


___


For 28, the answer is C. Three DNPH molecules will be needed to completely react.

http://i.imgur.com/vSkqtma.jpg
I messed it up a bit yesterday and forgot to draw the aldehyde leading to carboxylic acid.

This pic should explain the cleavage. If it's still unclear, let me know.

 

[Namehere]

Ooo I missed this question.

Right method, wrong answer. 2^3 = 8, not 6

The answer should be C.


___


For 28, the answer is C. Three DNPH molecules will be needed to completely react.

http://i.imgur.com/vSkqtma.jpg
I messed it up a bit yesterday and forgot to draw the aldehyde leading to carboxylic acid.

This pic should explain the cleavage. If it's still unclear, let me know.

Oh... 2^3 is indeed 8... my goodness i even said 2 x 2 x 2 = 6 :S This is what you get for not knowing how to multiply properly...

Q28 - I totally agree with your structure now! Before you missed the aldehyde turning into a carboxylic acid and the oxidation of the C at the double bond with the CH3 side chain.

Good work

 

[Muskan Achhpilia]

I have a doubt in the following questions of mcq can someone please help me...




Thank you so much!!