Which of the following underlined atoms is likely to be the strongest hydrogen bond acceptor? a)nitrogen (RNH2) b) Aniline nitrogen (ArNH2)
Hi Firyal,
Hydrogen bond acceptor which is measure of the basicity of molecule .
Aryl amines are considerabley less basic than alkylmines therefore alkylamines will be the strongest hydrogen bond acceptor. The, size and number of alkyl group will affect that little due to steric factors, hope this help you .Good lack
From quantum chemistry perspective, a direct way of determining the relative tendency to be a H-bond acceptor is examining electrostatic potential (ESP) on van der Waals surface corresponding to the lone pair region of the nitrogen. The more negative the ESP at that site, the stronger its tendency to be a H-bond acceptor. This kind of analysis can be easily done by using Gaussian or other quantum chemistry codes in combination with Multiwfn (http://sobereva.com/multiwfn). Gaussian can optimize the geometry and produce wavefunction information, while main function 12 of Multiwfn is able to locate all minima of ESP on van der Waals surface, see Section 4.12.1 of Multiwfn manual for example. An illustration is attached; as shown, RNH2 should be the stronger H-bond acceptor than ArNH2.
Firyal, There are experimental indicators in addition to theoretical indices.
A lot of data on H-bonding maybe found in this book (too expensive, I agree, but I can send you some more data) C. Laurence, J.-F. Gal, Lewis basicity and affinity scales: data and measurement; Wiley, Chichester, West Sussex; 2010. ISBN 978-0-470-74957-9.
There are thermodynamic (enthalpy=ΔH◦, Gibbs energy=ΔG◦) and spectroscopic H-bonding scales.
Using the 4-fluorophenol H-bond donor as reference, the -ΔG◦ (note the minus sign) of H-bonding formation are:
Primary alkylamines (various alkyl groups): approximate range 12 to 13 kJ/mol;
Anilines (various substituents on the ring) approximate range 1 to 3 kJ/mol.
Note that anilines form also weaker H-bonds by the pi (π) electrons.
So the weaker nitrogen basicity of anilines is experimentally observed.
Finally, XNH2 are also H-bond donors, in general weak, but not negligible for anilines.
- Badges
- Science topic