Chemistry: Post your doubts here!

[Saad the Paki]

Ummm I didn't even know branched chains were possible in free radical substitution. Can someone explain how the branched chain will form and why the answer is B (1 and 2 only)

 

[Rizwan Javed]

View attachment 59685
Ummm I didn't even know branched chains were possible in free radical substitution. Can someone explain how the branched chain will form and why the answer is B (1 and 2 only)

In free radical substitution any C-H bond can undergo, homolytic fission.

In 1, the C-H of Carbon 1 of one molecule has undergone this, and the Carbon 2 on the other molecule has undergone homolytic fission.
Similarly, in 2, both molecules' C-H of carbon 2 has undergone this.
But in 3, there are not two molecules of propane. But actually one molecule butane, and other of ethane. So this not correct as the question asks about the termination step in bromination of propane.


I have drawn boxes around the two free radicals which are involved in termination steps.

 

[Saad the Paki]

In free radical substitution any C-H bond can undergo, homolytic fission.

In 1, the C-H of Carbon 1 of one molecule has undergone this, and the Carbon 2 on the other molecule has undergone homolytic fission.
Similarly, in 2, both molecules' C-H of carbon 2 has undergone this.
But in 3, there are not two molecules of propane. But actually one molecule butane, and other of ethane. So this not correct as the question asks about the termination step in bromination of propane.

View attachment 59686
I have drawn boxes around the two free radicals which are involved in termination steps.

Thanks a lot !

 

[Saad the Paki]

I think both A and D are possible answerstandards but it's D in the ms. :/

 

[leenz98]

View attachment 59687
I think both A and D are possible answerstandards but it's D in the ms. :/

The bond dissocation energy is relevant to gasesous reactants.

 

[Saad the Paki]

The bond dissocation energy is relevant to gasesous reactants.

Oh didn't think of that! Thanks

 

[Saad the Paki]

Ik the answer is A but can someone tell me how the oxidation number of silver does not change?

 

[leenz98]

Oh didn't think of that! Thanks

I was doubting it so I googled it. Must not take all the credit.

 

[Saad the Paki]

I was doubting it so I googled it. Must not take all the credit.

Haha I appreciate your honesty

 

[Saad the Paki]

The answer is D. But the question asks what we can deduce from THESE observations. So how do we know that H2SO4 is a stronger oxidiser than iodine from these observations. And also, iodine is supposed to be a reducing agent and iodide is supposed to be an oxidising agent. It's the opposite here.

 

[Saad the Paki]

Anyone? The ans is A.

 

[usman]

A level chemistry by George Facer:

http://www.mediafire.com/download/3b65pyi9tr3eok3/A_level_-_Chemistry,_George_Facer.pdf

 

[Abdullahassan_99]

= (Mass no x Relative Abundance) / (Relative Abundance)

= {(10 * 1) + (11 * 4)} / 1 + 4

= 10.8

There is no peak for the mass no. 12, so we don't need to consider it. Note: Even if you consider it, it will be 12 * 0 = 0

THANK YOU !

 

[holoholo]

in 2014 may/june paper 42 mark scheme the optical isomers are given like this.



In the syllabus, it's given that potical isomers should be drawn like this.

My question is in the mark scheme why is the OH as a dashed line and then in the second isomer as a wedge but in the syllabus it is not ?

 

[The Sarcastic Retard]

View attachment 59711

in 2014 may/june paper 42 mark scheme the optical isomers are given like this.

View attachment 59713

In the syllabus, it's given that potical isomers should be drawn like this.

My question is in the mark scheme why is the OH as a dashed line and then in the second isomer as a wedge but in the syllabus it is not ?

Maybe they have drawn two possiblities. If u see that other part is not optical isomer. So its two possiblities, either optical isomer of left or of right

 

[qwertypoiu]

View attachment 59711

in 2014 may/june paper 42 mark scheme the optical isomers are given like this.

View attachment 59713

In the syllabus, it's given that potical isomers should be drawn like this.

My question is in the mark scheme why is the OH as a dashed line and then in the second isomer as a wedge but in the syllabus it is not ?

Both are fine. Key thing is to make sure the diagrams show the two molecules are non-superimposable.

Best to follow syllabus though.

 

[holoholo]

Are these complex ions ? and if so when we draw the structure how do we draw the dative bonding ?

 

[qwertypoiu]

Are these complex ions ? and if so when we draw the structure how do we draw the dative bonding ?

View attachment 59722

[PCl4]+ will be a tetrahedral structure, like CH4 or NH4+. I think showing the dative bonds is not important, but you could show one of the Cl atoms receiving a pair of electrons from P. (Just like N donates a lone pair to H in NH4+)

[PCl6]- will be an octahedral structure, like SF6. Again the coordination of the bonds shouldn't be important but you could show one Cl atom donating a pair of electrons to P.

 

[holoholo]

[PCl4]+ will be a tetrahedral structure, like CH4 or NH4+. I think showing the dative bonds is not important, but you could show one of the Cl atoms receiving a pair of electrons from P. (Just like N donates a lone pair to H in NH4+)

[PCl6]- will be an octahedral structure, like SF6. Again the coordination of the bonds shouldn't be important but you could show one Cl atom donating a pair of electrons to P.

How do we know if it is tetrahedral or square planar ?

 

[qwertypoiu]

Anyone? The ans is A.View attachment 59700

I'm guessing your trouble is with the right column.

• You know there is a cyclohexene.
• You know there are 20 carbon atoms.
• You know there is an aldehyde group.

So you can deduce this:


The molecular formula for the above structure is C20H36O
We have 36-28 = 8 too many hydrogen atoms. By adding a double bond to the aliphatic chain on the right side, we can reduce the number of H atoms.
For every double bond added, TWO hydrogen atoms are removed. So we need to add 8/2 = 4 double bonds. It will look like this:


So that makes a total of 5 C=C double bonds.

P.S. the actual structure of 11-cis retinal is a bit different; it is a structural isomer of above but that doesn't really matter.