JEE Main & Advanced Chemistry Haloalkanes and Haloarenes / हैलोएल्केन तथा हैलोएरीन्स Organometallic Compounds

Organic compounds in which a metal atom is directly linked to carbon or organic compounds which contain at least one carbon-metal bond are called organometallic compounds.

Example : Methyl lithium \[(C{{H}_{3}}Li)\], Dialkyl zinc \[({{R}_{2}}Zn)\], Alkyl magnesium halide \[(R-Mg-X)\]

(1) Methyl lithium :

\[\underset{\text{Methyl}\,\text{iodide}}{\mathop{C{{H}_{3}}I}}\,+2Li\underset{-10{}^\circ C}{\mathop{\xrightarrow{\text{Ether}}}}\,\underset{\text{Methyl}\,\text{lithium}}{\mathop{C{{H}_{3}}Li}}\,+LiI\]

• High reactivity of \[C{{H}_{3}}Li\] over grignard reagent is due to greater polar character of \[C-Li\] bond in comparison to \[C-Mg\] bond.

Chemical properties

(i) \[C{{H}_{3}}-Li+H\cdot OH\xrightarrow{{}}C{{H}_{4}}+LiOH\]

\[\xrightarrow{{{H}_{2}}O}C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}OH+LiOH\]

(iii) \[C{{H}_{3}}-Li+C{{O}_{2}}\xrightarrow{{}}C{{H}_{3}}\overset{O}{\mathop{-\overset{||}{\mathop{C}}\,-}}\,O-Li\]\[\xrightarrow{{{H}_{2}}O}C{{H}_{3}}COOH+LiOH\]

(iv) \[C{{H}_{3}}-Li+H\underset{H}{\mathop{-\underset{|}{\mathop{C}}\,=}}\,O\xrightarrow{{}}C{{H}_{3}}C{{H}_{2}}-O-Li\]\[\xrightarrow{{{H}_{2}}O}C{{H}_{3}}C{{H}_{2}}OH+LiOH\]

• Unlike grignard reagents, alkyl lithium can add to an alkenic double bond.

 \[R-Li+C{{H}_{2}}=C{{H}_{2}}\xrightarrow{{}}R-C{{H}_{2}}-C{{H}_{2}}-Li\]

(2) Dialkyl zinc : First organometallic compound discovered by Frankland in 1849.

 \[2RI+2Zn\underset{C{{O}_{2}}}{\mathop{\xrightarrow{Heat}}}\,2R-Zn-I\underset{C{{O}_{2}}}{\mathop{\xrightarrow{Heat}}}\,\underset{\text{Dialkyl}\,\text{zinc}}{\mathop{{{R}_{2}}Zn}}\,+Zn{{I}_{2}}\]

 Chemical properties

 Preparation of quaternary hydrocarbon :

 \[{{(C{{H}_{3}})}_{3}}CCl+{{(C{{H}_{3}})}_{2}}Zn\xrightarrow{{}}\underset{\text{Neopentane}}{\mathop{{{(C{{H}_{3}})}_{4}}C}}\,+C{{H}_{3}}ZnCl\]

 (3) Grignard reagent : Grignard reagent are prepared by the action of alkyl halide on dry burn magnesium in presence of alcohol free dry ether.

 Dry ether dissolves the grignard reagent through solvation.

 Grignard reagents are never isolated in free sate on account of their explosive nature.

 For given alkyl radical the ease of formation of a grignard reagent is, Iodide > Bromide > Chloride Usually alkyl bromides are used.

 For a given halogen, the ease of formation of grignard reagent is,\[C{{H}_{3}}X>{{C}_{2}}{{H}_{5}}X>{{C}_{3}}{{H}_{7}}X..........\]

 Since tertiary alkyl iodides eliminate HI to form an alkene, tertiary alkyl chlorides are used in place of tertiary alkyl iodides.

• Grignard reagent cannot be prepared from a compound which consists in addition to halogen, some reactive group such as \[-OH\] because it will react rapidly with the grignard reagent.

 The \[C-Mg\] bond in grignard reagent is some what covalent but highly polar.

 The alkyl group acts as carbanion. The majority of reaction of grignard reagent fall into two groups:

 (i) Double decomposition with compound containing active hydrogen atom or reactive halogen atom

 \[RMgX+HOH\xrightarrow{{}}RH+Mg(OH)X\]

 \[RMgX+{{D}_{2}}O\xrightarrow{{}}RD+Mg(OD)X\]

 \[RMgX+R'OH\xrightarrow{{}}RH+Mg(OR')X\]

 \[RMgX+R'N{{H}_{2}}\xrightarrow{{}}RH+Mg(R'NH)X\]

 \[RMgX+R'I\xrightarrow{{}}R-R'+MgIX\]

 \[RMgX+ClC{{H}_{2}}OR'\xrightarrow{{}}RC{{H}_{2}}OR'+MgClX\]

 (ii) Addition reaction with compounds containing