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Chemistry: Post your doubts here!

syed1995

syed1995

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Alice123 said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s11_qp_23.pdf
need help in Q4c....
Click to expand...


Only Part i was what i found to be difficult..


CH3(CH2)4CH(OH)CH=CHCHO


The double bond will be broken .. since both ends have 1 alkyl group and 1 h attached .. Aldehyde will be produced as both the products..

First part CH3(CH2)4CH(OH)CH

Becomes CH3(CH2)4CH(OH)CHO

since CHO can be oxidized further..

CH3(CH2)4CH(OH)COOH

since OH can be oxidized as well..

CH3(CH2)4C(O)COOH First Product

Second Part .. CHCHO

It becomes Aldehyde at both ends .. HOC-CHO

Further oxidizing gives us .. HOOC-COOH Second Product..

Part ii

PCl3 only reacts with OH. so replace OH with Cl..

CH3(CH2)4CH(OH)CH=CHCHO becomes CH3(CH2)4CH(Cl)CH=CHCHO

Part iii

It is a reducing agent.. the only thing which can be reduced is the aldehyde at the end .. since aldehyde gets reduced to primary alcohol..

CH3(CH2)4CH(OH)CH=CHCHO becomes CH3(CH2)4CH(OH)CH=CHCH2OH
 
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littlecloud11

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PhyZac said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w08_qp_4.pdf

Question 4 (b)...i cant get it!


Question 9, last part, working the structure from NMR,
i got CH3COCH2NH2 but in markscheme it is...CH3CH2CONH2 how?
Click to expand...

bi) C40H82 --------> C16H34 + C24H48
C16H34 is the fixed product you can choose any other hydrocarbon to be the second product as long as the number of C and H remains constant.

bii) Condition for cracking is Al2O3 catalyst and high temp.

biii) when you draw an alkane structure like propane: C-C-C you can see that the number of C-C bond is always one short of the number of Carbons present in the molecule. For an alkene like butene: C-C=C-C you can see that the number of C-C bond is two short of the number of carbons present.
The C-H bonds is the same as the number of hydrogen present in the molecule.
For C40H82 there are 39 C-C bonds and 82 C-H bonds
For C16H34 there are 15 C-C bonds and 34 C-H bonds
For C24H48 (alkene) there are 22C-C bonds, 1C=C bond and 48 C-H bonds
you can see that the number of C-H bonds broken and formed are exactly the same, so you don't have to consider these while calculting the enthalpy change.
More C-C bonds are broken than formed, the overall number is 39-15-22= 2 C-C bonds
And 1 C-C bond is formed.
ΔH = heat in -heat out
= 2* 350 - 610 = +90 Kj/mol

your value can differ from this answer depending on the hydrocarbon you chose as the second product but it is supposed to be a multiple of 90

iii) the positive ΔH shows that it's an endothermic reaction, and hence the high temp for the reaction.
 
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littlecloud11 said:
bi) C40H82 --------> C16H34 + C24H48
C16H34 is the fixed product you can choose any other hydrocarbon to be the second product as long as the number of C and H remains constant.

bii) Condition for cracking is Al2O3 catalyst and high temp.

biii) when you draw an alkane structure like propane: C-C-C you can see that the number of C-C bond is always one short of the number of Carbons present in the molecule. For an alkene like butene: C-C=C-C you can see that the number of C-C bond is two short of the number of carbons present.
The C-H bonds is the same as the number of hydrogen present in the molecule.
For C40H82 there are 39 C-C bonds and 82 C-H bonds
For C16H34 there are 15 C-C bonds and 34 C-H bonds
For C24H48 (alkene) there are 22C-C bonds, 1C=C bond and 48 C-H bonds
you can see that the number of C-H bonds broken and formed are exactly the same, so you don't have to consider these while calculting the enthalpy change.
More C-C bonds are broken than formed, the overall number is 39-15-22= 2 C-C bonds
And 1 C-C bond is formed.
ΔH = heat in -heat out
= 2* 350 - 610 = +90 Kj/mol

your value can differ from this answer depending on the hydrocarbon you chose as the second product but it is supposed to be a multiple of 90

iii) the positive ΔH shows that it's an endothermic reaction, and hence the high temp for the reaction.
Click to expand...
Thank you sooo much !!! b(iii) was troubling me the most, but i can now visualize it...THANKS ALOT Jazaki Allah khairan...Thank u thank u!
May Allah S.W.T have mercy on you and your family, and grant you the highest results in this life and hereafter. Ameeen. May Allah S.W.T support you, help you and guide you to be one best believers and grant you Jannatul Firdous Ameen.
 
littlecloud11

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PhyZac said:
Thank you sooo much !!! b(iii) was troubling me the most, but i can now visualize it...THANKS ALOT Jazaki Allah khairan...Thank u thank u!
May Allah S.W.T have mercy on you and your family, and grant you the highest results in this life and hereafter. Ameeen. May Allah S.W.T support you, help you and guide you to be one best believers and grant you Jannatul Firdous Ameen.
Click to expand...

I forgot about 9, do you still need that?
 
littlecloud11

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PhyZac said:
Oh yes I do need, if possible.
Click to expand...

Sure. Okay, first, the chemical shift values are not definite. The triplet near the region one might be due to R-CH3 (chemical shift .9) or R-CH2-R (chemical shift 1.3). Next, the quartet can be due to R-C(O)-CH3 (chemical shift 2.1) and lastly the small hump can be caused by R-C(O)-NH- (chemical shift (5-12)
Now, the first peak is a triplet, this means that the adjacent carbon to the carbon responsible for this peak has 2 hydrogen attached to it [using the n+1 rule]. So the first peak is likely to be R-CH3, where the R group is a -CH2- group. The second peak is a quartet and this is because the -CH2- group has the -CH3 group adjacent to it and so using the n+1 rule the no of sub-peak is 4 and this gives rise to the quartet. So the first part of the molecule is CH3-CH2-
The small hump doesn't have any definite peak, it means that the carbon adjacent to this group has no hydrogen attached to it. In R-C(O)-NH- there is no hydrogen attached to the carbon, so the third peak is caused by this group.
The reason you chose R-C(O)-CH3 is because it has a chemical shift value that fits with the second peak but it doesn't fit into the overall structure of the molecule as you can't attach an NH2 group to it.

the overall structure is therefore CH3-CH2-C(O)-NH2

If this seems too confusing, feel free to ask questions.
 
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littlecloud11 said:
Sure. Okay, first, the chemical shift values are not definite. The triplet near the region one might be due to R-CH3 (chemical shift .9) or R-CH2-R (chemical shift 1.3). Next, the quartet can be due to R-C(O)-CH3 (chemical shift 2.1) and lastly the small
....................
the overall structure is therefore CH3-CH2-C(O)-NH2

If this seems too confusing, feel free to ask questions.
Click to expand...
THanks Alot again, it was really mind consuming, yet i got it, and the point which satisfy me most was that chemical shift are not definite, i dont really have any questions, Thnks Alot Again!!!
Jazaki Allah Khairan, May Allah S.W.T shower His blessings upon you, and grant you and your parent happiness and success. Aameen.
 
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