Haloalkanes

Q1) Compounds with double bonds between carbon atoms can exhibit geometrical isomerism.
i) Draw structures for the two geometrical isomers of 1,2dichloroethene.
ii)What features of the double bond prevents isomer 1 from changing into isomer 2?

Q2) When 2-chloropropane reacts with sodium hydroxide two different reactions occur.
Each different reaction produces a different organic product
i) Outline a mechanism for Reaction 1 and state the role of the hydroxide ion in this reaction.
ii) Outline a mechanism for Reaction 2 and state the role of the hydroxide ion in this reaction.

The role of the hydroxide ion i think is a nucleophile in one reaction and a base in the other. I think both of these questions refer to substitution
i do not want the answer straight a way but a guide to start i can draw the compound above for q1) not sure what the isomer is though is it a cis trans isomer

Reply 1

The Northern Star

Hi Badeninc

Q1) i) By having "double bond" in the question, you can immediately think to youself that it is cis-trans isomerisation with two of the same group or atom on both side of the double bond.
ii) This is hard to explain without telling you the answer, so I'll give you the answer and explain it. It is the restricted rotation of the C=C double bond that stops it from changing into isomer 2. If you compare single to double bonds, single bonds can be rotated 360 degrees, such as a normal ordinary alkane chain (Butane for example) any of the C-C single bonds can rotate. But C=C bond have a "sigma" bond and a "pi" where electrons are delocalised cross the double bond. Though I am unsure if you have learnt that yet.

Q2) This question is trying to lead you into the "substitution" and the "elimination" of Halogenoalkanes using NaOH. Hint: The reaction takes place on the "chloro" part of the compound. The two reactions will occur in different conditions. You are correct on that one of the mechanism is Nucleophilic Substitution. The two reactions favour different conditions, one prefer Hot in Ethanol whereas the other prefer Cold in Water.

I apologise if it's too vague, but if I say anymore, I'll be giving you the answers straight away. All the best.

Reply 2

badeninc

Yes for Q1ii) We have learned about pi-bonds and rotation that makes sense now
Q2i) Substitution:

H3CCHClCH3 + NaOH ----) H3CCHOHCH3 + NaCl

Is that right 2chloropropane to propanol

Reply 3

Kyalimers

Yh substitution is correct for one of the reactions but i believe the other one is elimination is it not???

Reply 4

The Northern Star

Hi all

Yes, you are correct on Q2 i) The OH- from the NaOH will substitute the Cl from Chloropropane and an alcohol is produced with NaCl. The other mechanism is as sohanshah stated, an eliminiation reaction. The The end products is an alkene where in this case is Propene, NaCl and water:

C3H7Cl + NaOH ---> C3H6 + NaCl + H2O

However, you must note that elimination and substitution happens under different conditions. If i remembered correctly, Substitution favours 50/50 mixture of ethanol and water, because everything will dissolve in that, under heat and reflux. Eliminiation is in Ethanol solvent only, under heat and reflux. Hope this helps.

Haloalkanes

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