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Problem on dipole moment and H bond in organic compounds

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1. For cis-but-2-ene, it is said that the CH3-group is electron releasing so there is a net dipole moment pointing towards the carbon atoms of the double bond. In fact what is meant by "electron releasing" and why is CH3-group so?

 

2. For cis-butenedioic acid, there is intramolecular H bond between the O in C=O group and the H in O-H group. Why is the O in C=O group involved in the H bond but not the O in the O-H group?

 

Thank you so much!

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cis - but-2- ene has C2V symmetry so will have a small dipole moment due to the methyl groups slight positive inductive effect

 

cis but-2-enedioic acid will form an intramolecular hydrogen bond forming a 7 membered ring. There are a number of effects here which will determine which oxygen participates in the hydrogen bond. These include hybridisation of the lone pair orbital, ring strain, eclipsing cost, cost of twisting the conjugated system out of allignment and doubtless other effects too

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I see what you mean:D

 

I have another question. My teacher said that Sn1 nucleophilic substitution at saturated carbon can only be applied on tertiary haloalkane or secondary holoalkane, but not primary haloalkane. Why?

 

Also there is electrophilic substitution of aromatic compounds, but why isn't there electrophilic substitution of alkene?

 

Thanks!

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The Sn1 nucleophilic substitution cannot be applied to a primary haloalkane because of the instability of the carbocation that would be formed (i.e. a tertiary carbocation is much more stable than a primary carbocation).

 

Similarly for electrophilic substitution, this occurs on aromatic rings and not alkenes because of the stability of the resulting arenium ion (i.e. due to resonance). On an alkene, no such resonance can occur, and thus it is less stable.

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The Sn1 nucleophilic substitution cannot be applied to a primary haloalkane because of the instability of the carbocation that would be formed (i.e. a tertiary carbocation is much more stable than a primary carbocation).

 

Similarly for electrophilic substitution, this occurs on aromatic rings and not alkenes because of the stability of the resulting arenium ion (i.e. due to resonance). On an alkene, no such resonance can occur, and thus it is less stable.

 

In fact why is tertiary carbocation more stable than primary carbocation?

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Mainly due to hyperconjugation (i.e. the interaction of electrons from the carbon next to the carbocation will tend to have a stabilizing effect on the cation).

 

In addition, benzylic and allylic carbocations are even more stable.

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