Urgent chem test

Why there are many reactions of C=C double bond though it involves higher enthalpies than C-C single.

A double bond contains a sigma bond and a pi bond, while a single bond contains only a sigma bond. The average bond energy of a C=C single bond and that of a C-C double bond are 610 and 346 kJ/mol respectively. Therefore, the calculated average bond enthalpy of a C-C sigma bond and that of a C-C pi bond are 346 and 264 kJ/mol respectively. Obviously, a C-C sigma bond is stronger than a C-C pi bond.

In most reactions involving a C=C double bond, only the weaker C-C pi bond is broken and the stronger C-C is left unchanged.

There are much fewer reactions involving a C-C single bond because breaking the C-C sigma bond needs more energy.


Why R-O^- ions is more basic than OH- ?

Explanation 1:
H2O (the conjugate acid of OH^-) is more acidic than ROH (the conjugate acid of R-O^-), as H2O can protonate the base NH3 (to give NH4+) while ROH cannot. The stronger the conjugate acid, the weaker is the base. Therefore, OH^- is less basic than RO^-.

Explanation 2:
-R is an electron-releasing group. Therefore, R-O^- ion is destabilized, by inductive effect as the negative charge becomes more concentrated on the O atom. Therefore, the less stable R-O^- has a greater tendency to accept a proton.


Why bulky haloalkane is favored to elimination with strong base but not Sn1 reaction take place ( You know bulky carbocation is stable ) ?

Considering the reaction between (CH3)3CBr, a haloalkane with bulky alkyl groups, and a stronger base B^-.

Elimination is favored because the reaction with the attacking of B^- to one of the H atom in the three -CH3 groups, and this is easy to do so as there are as many as 9 H atoms to be attacked.

It is hardly to undergo SN1 reaction because the SN1 reaction involves the attacking of B^- to the carbocation (CH3)3­C^+, and this is almost impossible to do so as the positively-charged C atom is surrounded by 3 bulky -CH3 groups.

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