Orgo alway confusing this part

[299678]

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Anyone can explain this for each questions? so confusing.

1. The greatest amount of energy is released by the oxidative cleavage of an alkene that is
A. Unsubstituted
B. Monosubstituted
C. Disubstituted
D. Trisubstituted

2. The greatest amount of energy is required to break which of the following carbon carbon bonds?
A. H3C-CH3
B. (H3C)3-C(CH3)3
C. H2C=CH2
D. (H3C)2C=C(CH3)2

3. Which has the lowest ∆H of combusting a CH2 group?
a) Cyclohexane
b) Cyclopentane
c) Cyclopropane
d) Cyclopentane
e) Cyclooctane

4. Which halogen is the most endothermic first propagation step during alkane haogenation?

a) Cl2 b) I2 c) Br2 d) F2 e) At2

 

[TexasOMFS]

1. A.
Just think about which alkene is most stable, as this one will release the least amount of energy when oxidatively cleaved. In order of stability Tri>Di>Mono>Un. Unsubstituted is the least stable (most reactive), therefore it releases the most energy.
2. D.
Kind of the same concept as last one. You know that it takes more energy to break a double bond than a single bond so that narrows your choices to C & D. It'll take more energy to break the tetrasubstituted alkene because it is more stable than the unsubstituted one.
3. A.
Cyclohexane is the most stable of the molecules listed because in it's chair conformation there is zero angle, steric, and torsional strain.
4. C?
Not sure about this one. Tell me if it's right and I'll explain my reasoning.

 

[JBarr29]

1. A.
Just think about which alkene is most stable, as this one will release the least amount of energy when oxidatively cleaved. In order of stability Tri>Di>Mono>Un. Unsubstituted is the least stable (most reactive), therefore it releases the most energy.
2. D.
Kind of the same concept as last one. You know that it takes more energy to break a double bond than a single bond so that narrows your choices to C & D. It'll take more energy to break the tetrasubstituted alkene because it is more stable than the unsubstituted one.
3. A.
Cyclohexane is the most stable of the molecules listed because in it's chair conformation there is zero angle, steric, and torsional strain.
4. C?
Not sure about this one. Tell me if it's right and I'll explain my reasoning.

I'm thinking C to...Bromine is the only one thats not exothermic to my knowledge - its definitely not Cl but im curious as to what the answer is.

 

[299678]

Correct answer is
1, A
2. D
3. A
4. B
This is why I am always confusing whenever I face with this sort of question
Anyway, I was pretty sure about #4 though. F is too reactive, I is relatively not reactive.
Learned that concept when we do radical bromination

 

[JBarr29]

You're right it is Iodine.
The propagating steps in the bromination of F are highly exothermic, therefore reacting very quickly. As we move down the group, reactivity becomes slower. Bromine is endothermic, and Iodine is even more endothermic.

If you have the destroyer take a look at question 78 Ochem section.

 

[sfoksn]

why can't At be the answer? it's even more below than I...

 

[AmpedUp]

why can't At be the answer? it's even more below than I...

Quite simply, it can't undergo alkane halogenation...Br2 and Cl2 are pretty much the only ones...I2 is almost nothing and F2 happens sometimes (though it's the most reactive).

 

[TexasOMFS]

I don't even know if At is considered a halogen, even though it's in the same group...we don't consider it at all through all of GC and OC...plus it's radioactive lol.

 

[kpanesar]

yeah the last one for free radical reactions it goes

exothermic to endothermic
F > Cl > Br >I

 

[UndergradGuy7]

I just watched this on chad's videos yesterday. F reacts violently and is never used. Cl2 is exothermic that is why it adds Cl to more than one spot and gives a mixture of products.

Br2 is endothermic, since it needs energy (from light,etc) it is harder for it to add Br to a spot and is more selective. It will only add Br to one spot (the most substituted).

Going in this pattern I2 is next after Br2 and will be even more endothermic.

At is never used in organic chemistry radical reactions. At least I have never seen it.