need help on CHEM mCqS

[hassam]

 

[hassam]

anyone........?

 

[XPFMember]

anyone........?

Assalamoalaikum!!

i tried...but i dont get it

if i reach a conclusion...will let u know

 

[hassam]

k

 

[DragonCub]

I get it! The answer to Q20 is C.
First, it is -cis so B and D are wrong.

There is a regular change that, when each C-C bond is replaced by a C=C bond, the total number of H will decrease by 2.
Then, suppose an saturated alkane containing 20 carbon atoms. The number of H atoms it has is 20 * 2 + 2 = 42. The compound in the question has only 28 H. There are 14 deducted. And note that 2 of them are due to replacing an end's methyl by an aldehyde group. So 12 H atoms are deducted by changing C-C to C=C. 12 / 2 = 6. There are 6 C=C double bonds.
Answer is C.

 

[DragonCub]

For Q21 you can first set a C atom as the chiral centre.
Then, to assure the four gruops linked to it are all different, the only way which uses fewest C atoms is: H, CH3, C2H6 and C3H8.
So count the number of C atoms in this alkane: 1 in CH3, 2 in C2H6, 3 in C3H8, and the one as chiral centre, total 7.
Answer is A.

 

[hassam]

http://commons.wikimedia.org/wiki/File: ... uselang=et

 

[hassam]

in 11 cis-retinal wat abt the cyclohexene ring...?

 

[hassam]

 

[soumayya]

The answer is B...replace 1 R group by H since an aldehyde contains an H atom bonded to the C of the -CO group...replace the -CN group by -CH2CO2CH3 in the second molecule since it behaves lyk -CN...

hpe zat hlps...

 

[hassam]

but y not C....ms also says C

 

[DragonCub]

in 11 cis-retinal wat abt the cyclohexene ring...?

Oops, very sorry I didn't notice the "ring" in the question. Cyclo ring deducts further 2 H atoms... So the number of C=C is 5, answer is A. Sorry about that!

Well I think the answer to Q27 is C indeed. The attached image would explain it.

 

[soumayya]

hmm...yh it's C...i didn't draw the full structural formula...hence i failed to spot the isomers...