Chemistry: Post your doubts here!

[Wonder girl]

Can anyone plz upload A Level Chemistry Revision Guide by David Bevan

 

[JD REBORN]

MCQ 27 and 28 with explanation.

 

[Doctor Nemo]

MCQ 27 and 28 with explanation.

27)

It took me a while to come to the answer of C. There is no denying that if the two molecules are hydrolysed that the products will be the same. However alkenes are normally said to undergo electrophilic addition. So what is wrong with A? The addition of bromine occurs in two steps. The first step is clearly electrophilic addition to form a positive ion. The second step however is addition of a nucleophile either Br- or OH-.

This seems to be more of a question of word play than organic chemistry. However that is often the case with MCQ. This question is a good example of why you have to practice the past papers so that if you encounter this question again or something like it you will not be confused by the choices.

28)

This is easier. If it is not reactive with mild oxidizing agents then it must be a tertiary alcohol. The -OH group must be on the carbon next to the benzene ring. Dehydration (this word implies that it is an alcohol) would give product D.

 

[Doctor Nemo]

27)

It took me a while to come to the answer of C. There is no denying that if the two molecules are hydrolysed that the products will be the same. However alkenes are normally said to undergo electrophilic addition. So what is wrong with A? The addition of bromine occurs in two steps. The first step is clearly electrophilic addition to form a positive ion. The second step however is addition of a nucleophile either Br- or OH-.

This seems to be more of a question of word play than organic chemistry. However that is often the case with MCQ. This question is a good example of why you have to practice the past papers so that if you encounter this question again or something like it you will not be confused by the choices.

28)

This is easier. If it is not reactive with mild oxidizing agents then it must be a tertiary alcohol. The -OH group must be on the carbon next to the benzene ring. Dehydration (this word implies that it is an alcohol) would give product D.

Actually I reread the question and here again is why MCQ is difficult and you have to read the questions like a lawyer reading a contract.

When I read choice A I was thinking electrophilic addition reaction because that is what alkenes do. However the choice is electrophilic substitution which is clearly incorrect because it is an addition reaction.

 

[JD REBORN]

27)

It took me a while to come to the answer of C. There is no denying that if the two molecules are hydrolysed that the products will be the same. However alkenes are normally said to undergo electrophilic addition. So what is wrong with A? The addition of bromine occurs in two steps. The first step is clearly electrophilic addition to form a positive ion. The second step however is addition of a nucleophile either Br- or OH-.

This seems to be more of a question of word play than organic chemistry. However that is often the case with MCQ. This question is a good example of why you have to practice the past papers so that if you encounter this question again or something like it you will not be confused by the choices.

28)

This is easier. If it is not reactive with mild oxidizing agents then it must be a tertiary alcohol. The -OH group must be on the carbon next to the benzene ring. Dehydration (this word implies that it is an alcohol) would give product D.

i dont understand ur explanation of 27(ur writing font is small)

 

[Doctor Nemo]

i dont understand ur explanation of 27(ur writing font is small)

Sorry, I myself got confused with question 27. I looked at it for 5 minutes and did not read it correctly.

Since the question is about alkenes I was thinking about electrophilic addition and I read choice A as electrophilic addition. Possibly because right below it is the word addition. However choice A is electrophilic substitution which not correct for this reaction-because it is an addition reaction. Choice C must be correct since substitution of the Br on the two molecules with -OH will give the same molecule.

 

[omg]

anybody having papers of 2000 and 2001?????? even older 1999 or 1998?????? pls if u do give me the linkkk !!!! p4!!

 

[JD REBORN]

Q21)

 

[abcde]

Q21)

21) Both are primary alcohols and would form carboxylic acids upon oxidation so acidified potassium dichromate (VII) reacts the same with both isomers.
The reactions with sodium and phosphorus pentachloride will also be the same for both since these reactions do not matter on the compounds being different isomers. The reaction with sodium depends on the electromagnetic nature of the oxygen atom in -OH, which is the same for both isomers. In the case of PCl3, the nucleophilic does not depend on the different groups attached to the functional group carbon.
Dehydration with conc. H2SO4 is different for both since in the first alcohol, the functional group carbon is immediately attached to a carbon atom linked with 3 methyl groups. This provides a comparively larger electron cloud than the 2nd alcohol. The larger electron cloud makes the formation of an alkene and the detachment of a H+ ion faster.
So, B is correct.

 

[JD REBORN]

22 and 24

 

[Doctor Nemo]

22 and 24

For 22 choice A is not correct because you could get substitution at both 1 and 2 positions. For B and D the two are dilute aqueous solutions which are not going to do much of anything. This leaves C which is the reaction of an alcohol with SOCl2 which is the preferred method for making alkyl chlorides.

For 24 the answer is to find the molecule where one of the carbons in the double bond has attached to it two identical groups. This is 2-methyl-2-pentene

CH3-C=CH-CH2-CH3
|
CH3

 

[JD REBORN]

For 22 choice A is not correct because you could get substitution at both 1 and 2 positions. For B and D the two are dilute aqueous solutions which are not going to do much of anything. This leaves C which is the reaction of an alcohol with SOCl2 which is the preferred method for making alkyl chlorides.

For 24 the answer is to find the molecule where one of the carbons in the double bond has attached to it two identical groups. This is 2-methyl-2-pentene

CH3-C=CH-CH2-CH3
|
CH3

ur drawing is wrong but anyway i get the idea

 

[bamteck]

When the electrolytic cell is stirred, two of the products react to form a solution. Write an equation for the reaction which occurs if the cell is kept cold.

State two everyday uses if the solution formed.

 

[bamteck]

This is the electrolysis of brine using inert electrodes !

 

[Doctor Nemo]

ur drawing is wrong but anyway i get the idea

ok, the structure did not show up correctly when I pasted it.

 

[Ahmed Ali Akbar]

i need help in may june 11 P1 ...mcq 22, 6, 35..

 

[soumayya]

i need help in may june 11 P1 ...mcq 22, 6, 35..

paper 11 or 12..??

 

[shimeka]

If anyone can answer this question for me please...

Which mass of solid residue can be obtained from the thermal decomposition of 4.10g of anhydrous calcium nitrate?

 

[soumayya]

2 Ca(NO3)2 → 2 CaO + 4 NO2 + O2

Residue is CaO since NO2 and O2 are gases..

Mr of Ca(NO3)2 = 164.1
Mr of CaO = 56.1

From equation:
(2*164.1) g of Ca(NO3)2 decomposes into (2*56.1) g of CaO
Hence, 4.10 g of Ca(NO3)2 decomposes into 1.40 g of CaO

hpe it hlps...

 

[shimeka]

Thankss..