Organic Chem Questions

Charmy SG

hey, stuck with a few questions, would appreciate your help(:

it's A lvl chem, and it might be basic questions, so forgive me for taking up your time.

1) Is cyclohexene planar? If not, why? (then what about cyclohexane)

2) When ethene reacts with bromine in the prescence of concentrated aqueous sodium nitrate, the product contains the compound, CH2(ONO2)CH2Br. what is the intermediate formed during this reaction?

where did the (ONO2) even come from in the first place? and how would you know whether the (ONO2) or the (Br) is in the intermediate first?

3) When benzene undergoes electrophilic substitution with ICl, why is Iodine attached to form the intermediate instead of the Cl?

4)When Propene (CH3CHCH2) undergoes electrophilic addition with Br-OH, it becomes CH3CH(OH)CH2Br. Why is it not CH3CHBrCH2(OH)? is it because in this case Br acts like H and acts according the the markownikoff's rule? then what about if it's Br-Cl?

TYVM(:

UltimaOnline SG

Hi Charmy

hey, stuck with a few questions, would appreciate your help(:

it's A lvl chem, and it might be basic questions, so forgive me for taking up your time.

1) Is cyclohexene planar? If not, why? (then what about cyclohexane)

2) When ethene reacts with bromine in the prescence of concentrated aqueous sodium nitrate, the product contains the compound, CH2(ONO2)CH2Br. what is the intermediate formed during this reaction?

where did the (ONO2) even come from in the first place? and how would you know whether the (ONO2) or the (Br) is in the intermediate first?

3) When benzene undergoes electrophilic substitution with ICl, why is Iodine attached to form the intermediate instead of the Cl?

TYVM(:

>>> 1) Is cyclohexene planar? If not, why? (then what about cyclohexane) <<<

Cyclohexene contains only 1 double bond, with sp2 hybridized trigonal planar carbon atoms. All the other carbon atoms in cyclohexene (and all carbon atoms in cyclohexane) that have 4 sigma bonds are sp3 hybridized and tetrahedral.

>>> 2) When ethene reacts with bromine in the prescence of concentrated aqueous sodium nitrate, the product contains the compound, CH2(ONO2)CH2Br. what is the intermediate formed during this reaction?

where did the (ONO2) even come from in the first place? and how would you know whether the (ONO2) or the (Br) is in the intermediate first? <<<

Always draw the mechanism to visualize the reaction pathway. The nitrate(V) ion is a nucleophile that attacks the carbocation intermediate, that is formed after the nucleophilic pi-electrons of the alkene attacks the electrophilic bromine atom.

>>> 3) When benzene undergoes electrophilic substitution with ICl, why is Iodine attached to form the intermediate instead of the Cl? <<<

Because chlorine is more electronegative than iodine; hence the nucleophilic pi-electrons of benzene attack the iodine electrophile to form the tetrahedral intermediate (known as the Arrhenium ion or sigma complex).

>>> 4)When Propene (CH3CHCH2) undergoes electrophilic addition with Br-OH, it becomes CH3CH(OH)CH2Br. Why is it not CH3CHBrCH2(OH)? is it because in this case Br acts like H and acts according the the markownikoff's rule? then what about if it's Br-Cl? <<<

Don't blindly memorize Markovnikov's Rule. Understand why it works, then you can identify when it fails to work (a possible H2 exam question involves alkenes like 3,3,3,-trichloroprop-1-ene, for which Markovnikov's Rule fails).

Identify the electrophile and nucleophile in Br-OH. Draw both alternative mechanism pathways leading to the Markovnikov and anti-Markovnikov product. The more stable carbocation intermediate will result in the major product.

Note that carbocations, like alkyl radicals as well as alkene sp2 carbons, are all stabilized by a greater number of electron-donating 'R' groups (though each for different reaons which are beyond the H2/H3 syllabuses).

Charmy SG

Thank you for the reply!!

so, just to see if i got the Markovnikov's Rule right.

for question 4, the electrophile is the Br and the nucleophile is OH because OH is more electronegative than Br. so according the the rule, the nucleophile mostly forms bonds with the carbon atom which has the most bonds with electron-donating groups and carbon atoms(which makes it more stable), and the Br goes to the other less substituted carbon, forming the compound CH3CH(OH)CH2Br.

and if it's Br-Cl, it would be CH2CHClCH2Br instead.

UltimaOnline SG

Yes that's correct; though you should iterate specifically that the basis for Markonikov's rule has to do with comparing the relative stabilities of the (alternative pathways') carbocation intermediates; the terminal / primary carbocation intermediate is a lot less stable (hence leads to the minor product) than the non-terminal / secondary carbocation intermediate (which hence leads to the major product).