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AbstractTe basicity of amines depends on the ability of nitrogen

to donate its lone electron pair to water, forming the

hydroxide ion. Bulky atoms or molecules attached to

nitrogen may influence the ease of water bonding to

the amine. Computer models are performed to ascer-tain the structures of NH3, CH3NH2, (CH3)2NH, and

(CH3)3N, as well as the heats of formation (Hf) and

solvation energies (Hsol), to correlate theoretical results

to experimental pKbvalues.

IntroductionTe chemistry and bonding characteristics of nitrogen

are reviewed in Part 1.1Te basicity of amines is

governed by the ability of nitrogen to bond through

its lone electron pair to water. In doing so, a free

hydroxide ion (OH-

) forms, resulting in the observedbasicity. Part 2 discusses the use of computer models

to explain differences in basicity due to electron

donating or withdrawing groups bonded directly to the

nitrogen atom.2Atoms or molecules furnishing addi-

tional electron density to nitrogen results in more basic

amines. Localized charges on nitrogen (e.g., population

analysis) shows increasing partial negative charge on

nitrogen in the series methylamine < dimethylamine

CH3NH2(-19.49 kJ/mol) > (CH3)2NH (-19.61 kJ/mol) >

and (CH3)3N (-13.37 kJ/mol). Again, this is reflective of the

ease of H+transfer from H+-OH- to the bonding electrons

in either ammonia or CH3NH2, and the greater difficulty

in H+transferring to either (CH3)2NH or (CH3)3N.

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On the Hydration of Amines and Basicity continued

Figure 4. Inductive and steric effects and their influence onamine basicity

2

2.5

3

3.5

4

4.5

pKb

Amine

CH3NH2 (CH 3) 2NH

(CH 3) 3N

CH3CH2H2

(CH 3CH2) 2NH

(CH 3CH2) 3N

Steri c hindrance

Inductive effects

aking into account the inductive effects discussed in

Part 2, whereby additional alkyl groups add partial nega-

tive charge to the nitrogen atom, resulting in increased

basicity of amines and those results presented in this

study based on ease of hydration, the overall basicity

of amines follows the order: 2o> 1o> 3o> NH3. In 3o

amines, hydration stabilization is minimal and steric

hindrance is maximum, so 3oamines are the least basic

in spite of maximum inductive effect. In 1oamine,

steric hindrance is least, hydration is maximum, and the

inductive effect of the single alkyl group is minimum.

So, 1oamines are slightly more basic than 3oamines.

As a result of hydration, inductive and steric factors cause

2o amine to be more basic than either 1oor 3oamines.

Tese effects are summarized in able 2 and Figure 4.

Table 2. Summary of effects influencing amine basicity

Inductive

Effects

Steric

Hindrance

Hydration

Energy

Strength of

Amine

1 Amines Small Very small Large Intermediate

2 Amines Moderate Moderate Moderate Highest

3 Amines Large Very large Small Weakest

ConclusionsTe basicity of amines is controlled by a balance between

inductive, steric, and hydration effects. Te addition of

electron donating groups, such alkyl groups, contributes

partial negative charge to nitrogen, thereby increasing

the basicity of the molecule. Hydration of ammonia or

amines and the resultant basicity is determined by the

ease at which a H+is donated from water to the electron

pair on nitrogen. oo many bulky alkyl groups may

block the approach of water toward the nitrogen, thus

limiting effective H+transfer. Te

overall basicity of amines follows

the order: 2o> 1o> 3o> NH3.

References1. N.M. Martyak, Analyst, Vol. 18, No. 3, pg. 2 (2011).2. N.M. Martyak, Analyst, Vol. 19, No. 1, pg. 2 (2012).

3. Joyce J. Kaufman,International J. QuantumChem., Vol. 5, Issue S4, 205-8 ( Jan. 1970).

4. R.S. Mulliken,J. Chem. Phys., 23, 1833, 1841,2338, 2343 (1955).

5. R.S. Mulliken,J. Chem. Phys., 1934,2, 782.

6. Titan, Ver. 1.08, Wavefunction, Inc. (2001).

7. T. Clark,A Handbook of ComputationalChemistry, Wiley, New York 1986.

8. Spartan 08, Wavefunction, Inc., Irvine, CA(20062009).

9. M.B. Smith and J. March, Marchs Advanced

Organic Chemistry, Reactions, Mechanismand Structure, 5th Ed., John Wiley and Sons,NY, NY (2001)

10. J. McMurry, M. E. Castellion and D. S.Ballantine, Fundamentals of General,Organic, and Biological Chemistry, PrenticeHall, Inc. (2007).

Nicholas M. Martyak is with Taminco

Higher Amines Inc. and can be reached

at (610) 390-1156 or nick.martyak@

taminco.com.

mailto:nick.martyak@taminco.commailto:nick.martyak@taminco.comhttp://nmc-nic.com/mailto:nick.martyak@taminco.commailto:nick.martyak@taminco.com