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Thank you!

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

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!

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!