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Amines — Chemistry STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceChemistryAmines4 Marks
Question
Read the passage given below and answer the following questions:
The amines are basic in nature due to the presence of a lone pair of electron on $N-$atom of the $-NH_2$ group, which it can donate to electron deficient compounds. Aliphatic amines are stronger bases than $NH_3$ because of the $+I$ effect of the alkyl groups. Greater the number of alkyl groups attached to $N-$atom, higher is the electron density on it and more will be the basicity. Thus, the order of basic nature of amines is expected to be $3^\circ > 2^\circ > 1^\circ ,$ however the observed order is $2^\circ > 1^\circ > 3^\circ .$ This is explained on the basis of crowding on $N-$atom of the amine by alkyl groups which hinders the approach and bonding by a proton, consequently, the electron pair which is present on $N$ is unavailable for donation and hence $3^\circ$ amines are the weakest bases.
Aromatic amines are weaker bases than ammonia and aliphatic amines. Electron-donating groups such as $-\ce{CH_3, -OCH_3,}$ etc. increase the basicity while electron-withdrawing substitutes such as $\ce{-NO_2, -CN,}$ halogens, etc. decrease the basicity of amines. The effect of these substituents is more at $p$ than at $m-$positions.
The following questions are multiple choice questions. Choose the most appropriate answer:
  1. Which one of the following is the strongest base in aqueous solution?
  1. Methyl amine.
  2. Tri methyl amine.
  3. Aniline.
  4. Dimethyl amine.
  1. Which order ofbasicity is correct?
  1. Aniline $>$ $m-$toluidine $> o-$toluidine
  2. Aniline $> o-$toluidine $> m-$toluidine
  3. $o-$toluidine $>$ aniline $> m-$toluidine
  4. $o-$toluidine $<$ aniline $< m-$toluidine
  1. What is the decreasing order of basicity of primary, secondary and tertiary ethylamines and $NH_3$?
  1. $\ce{NH_3 > C_2H_5NH_2 > (C_2H_5)_2NH > (C_2H_5)_3N}$
  2. $\ce{(C_2H_5)_3N > (C_2H_5)_2NH_{ }> C_2H_5NH_2 > NH_3}$
  3. $\ce{(C_2H_5)_2NH > C_2H_5NH_2> (C_2H_5)_3N > NH_3}$
  4. $\ce{(C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3​​​​​​​}$
  1. The order of basic strength among the following amines in benzene solution is:
  1. $\ce{CH_3NH_2 > (CH_3)_3N > (CH_3)_2NH}$
  2. $\ce{(CH_3)_3N > (CH_3)_2NH > CH_3NH_2​​​​​​​}$
  3. $\ce {CH_3NH_2 > (CH_3)_2NH > (CH_3)_3N}$
  4. $\ce{(CH_3)_3N > CH_3NH_2 > (CH_3)_2NH}$
  1. Choose the correct statement.
  1. Methylamine is slightly acidic.
  2. Methylamine is less basic than ammonia.
  3. Methylamine is a stronger base than ammonia.
  4. Methylamine forms salts with alkalies.
✓

Answer

  1. $(d)$ Dimethyl amine.
The increasing order of basicity of the given compounds is $\ce{(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > C_6H_5NH_2.}$ Due to the $+I$ effect of alkyl groups, the electron density on nitrogen increases and thus, the availability of the lone pair of electrons to proton increases and hence, the basicity of amines also increases. So, aliphatic amines are more basic than aniline. ln case of tertiary amine $(CH_3)_3N,$ the covering of alkyl groups over nitrogen atom from all sides makes the approach and bonding by a proton relatively difficult, hence the basicity decreases. Electron withdrawing groups decrease electron density on nitrogen atom and thereby decreasing basicity.
  1. $(d)\ o-$toluidine $<$ aniline $< m-$toluidine
In general, electron donating $( +R)$ group which when present on benzene ring $\ce{(-NH_2, -OR, -R,}$ etc$.)$ at the para position increases the basicity of aniline.
Ortho substituted anilines are weaker bases than aniline due to ortho effect.
  1. $(d)\  \ce{(C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3}$
In case of ethylamines, the combined effect of $(c)$ inductive effect, steric effect and salvation effect gives the order of basic strength as
$(\text{C}_2\text{H}_5)_2\text{NH}>(\text{C}_2\text{H}_5)_3\text{N}>\text{C}_2\text{H}_5\text{NH}_2>\text{NH}_3\\\ \ \ \ \ \ \ \ (2^\circ)\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (3^\circ)\ \ \ \ \ \ \ \ \ \ \ \ \ (1^\circ)$
  1. $(b)\ \ce{(CH_3)_3N > (CH_3)_2NH > CH_3NH_2​​​​​​​}$
In non$-$aqueous solvents the basic strength of alkyl amines follows the order:
tertiary amines $>$ secondary amines $>$ primary amines.
  1. $(c)$ Methylamine is a stronger base than ammonia.
Methyl amine is stronger base than ammonia due to electron releasing inductive effect of methyl group.

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Dependence of the rate of reaction on the concentration of reactants, temperature, and other factors is the most general method for weeding out unsuitable reaction mechanisms. The term mechanism means all the individual collisional or elementary processes involving molecules $($atoms, radicals, and ions included$)$ that take place simultaneously or consecutively to produce the observed overall reaction. For example, when hydrogen gas reacts with bromine, the rate of the reaction was found to be proportional to the concentration of $H_2$ and to the square root of the concentration of $Br_2.$ Furthermore, the rate was inhibited by increasing the concentration of $HBr$ as the reaction proceeded. These observations are not consistent with a mechanism involving bimolecular collisions of a single molecule of each kind. The currently accepted mechanism is considerably more complicated, involving the dissociation of bromine molecules into atoms followed by reactions between atoms and molecules:
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$($source: Moore, J. W., Pearson, $R. G. (1981).$ Kinetics and mechanism. John Wiley Sons.$)$
$(a).$ Predict the expression for the rate of reaction and order for the following:
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