18. Geometries for small molecules such as methyl Buoride can be determined from
ID: 578818 • Letter: 1
Question
18. Geometries for small molecules such as methyl Buoride can be determined from microwave spectroscopy, whereas vari- ous forms of diffraction-s-ray, electron or neutron diffraction can be applied to larger molecules. Shown below are bond lengths and angles for various methyl halides determined by various methods Rationalize the trend in the C-X bond lengths (angstroms).Also rationalize the H-C-Hand H-C-Xbond angles using VSEPR. Does an argument based upon electronegativities also explain the trends in bond angles? Molecule CH.F CH.CI CH.Br CHJ C-H bond length C-X bond length H-C-H bond angle H-C-X bond angle 1.09 1.096 1.11 10% 1.385 1.781 1.939 2.139 110.2 110.52 11.12 108.2 108.0 107.14 106.58Explanation / Answer
The C-X bond lengths increase in the order C-F < C-Cl < C-Br < C-I. This is due to the electronegativity and size of the X (halogen) atom. The electronegativity of the halogens decrease as F > Cl > Br > I.As the electronegativity of the halogen decreases, the ability of the halogen atom to withdraw the electron pair of the C-X bond to itself decreases. Consequently, the C-X bond becomes weaker. Moreover, the size of the halogens increase from F < Cl < Br < I. As the size of the halogens increases, the C-X bond becomes weaker. These two facts contribute to the fact that the C-F bond length is the shortest while the C-I bond is the largest.
Due to the large size of I atom, there is steric between the C and I atoms; consequently, the H-C-I bond angle in CH3I becomes slightly lower than 1090 while the H-C-H bond angle increases at the expense of H-C-I bond angle. F atom is the smallest atom in the periodic table and hence, the C atom is not interfered with the F atom and the H-C-F and H-C-H bond angles are close to the tetrahedral bond angle of 1090.
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