Alcohol, Phenol and Ether MCQ Questions & Answers in Organic Chemistry | Chemistry

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Hydroboration-oxidation reaction

Heavy metal ions, particularly $$A{g^ + },$$  catalyse $${S_N}1$$  reaction because of the presence of empty orbital.
\[C{{H}_{3}}C{{H}_{2}}Br+A{{g}^{+}}\rightleftharpoons {{\left[ C{{H}_{3}}C{{H}_{2}}\cdot \cdot \cdot X\cdot \cdot Ag \right]}^{+}}\xrightarrow[\left( -AgX \right)]{\text{slow}}C{{H}_{3}}CH_{2}^{+}\xrightarrow[\text{from}\,{{H}_{2}}O]{O{{H}^{-}}}C{{H}_{3}}C{{H}_{2}}OH\]

Such questions can be solved by considering the relative basic character of their conjugated bases which for $${H_2}O,{C_2}{H_2},{H_2}C{O_3}$$     and $${C_6}{H_5}OH$$   are $$^ - OH,HC \equiv C\mathop :\limits^ - ,HCO_3^ - ,{C_6}{H_5}{O^ - }$$
More the possibility for the dispersal of the negative charge, weaker will be the base. Thus the relative basic character of the four bases is
$$\underset{\text{Due to resonance}}{\underbrace{\underset{\begin{array}{l}\text{Eqivalent}\\ \text{resonating}\end{array}}{HC{O}_3^{-}}<\underset{\begin{array}{l}\text{Non-equivalent}\\ \text{structures}\end{array}}{C_6{H}_5{O}^{-}}}}<\underset{\begin{array}{l}\text{Oxygen can accommodate}\\ \text{negative charge easily}\end{array}}{{}^{-}OH<HC\equiv C^{-}}$$
Thus the acidic character of the four corresponding acids will be
\[{{H}_{2}}C{{O}_{3}}>{{C}_{6}}{{H}_{5}}OH>{{H}_{2}}O>HC\equiv CH\]

When fert - alkyl halides are used in Williamson synthesis elimination occurs rather than substitution resulting into formation of alkene. Here alkoxide ion abstract one of the $$\beta - $$  hydrogen atom along with acting as a nucleophile.

Whenever dehydration can produce two different alkenes, major product is formed according to Saytzeff rule i.e. more substituted alkene ( alkene having lesser number of hydrogen atoms on the two doubly bonded carbon atoms ) is the major product.
Such reactions which can produce two or more structural isomers but one of them in greater amounts than the other are called regioselective ; in case a reaction is 100% regioselective, it is termed as regiospecific.
In addition to being regioselective, alcohol dehydrations are stereoselective ( a reaction in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amount than any other ).

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