Carbon-Hydrogen Stretching sp3 hybridized,one-fourth s character 2800-3000cm-1 sp2 hybridized,one-third s character 3000-3100cm-1 H 一C≡C一H sp hybridized,one-halfs character 3300 cm-1 (sharp) Copyright2010 Pearson Prentice Hall,Inc. A greater percent of s character in the hybrid orbitals will make the C-H bond stronger. An sp3 hybridized carbon has a 25%s character,an sp2 has around 33%s character,and an sp carbon has 50%s character. The C-H bond of an sp3 carbon will be slightly weaker than the C-H of an sp2 or an sp carbon. Chapter 12 16
Chapter 12 16 Carbon–Hydrogen Stretching • A greater percent of s character in the hybrid orbitals will make the C—H bond stronger. • An sp3 hybridized carbon has a 25% s character, an sp2 has around 33% s character, and an sp carbon has 50% s character. • The C—H bond of an sp3 carbon will be slightly weaker than the C—H of an sp2 or an sp carbon
IR Spectrum of Alkanes wavelength (um) 3.5 5.5 13 14 1516 CH CH CH, CH(CH3)CH n-octane 2500 200018001600140012001000 800 60 wavenumber(cm) Copyright010 Pearson Prentice Hall,Inc. An alkane will show stretching and bending frequencies for C-H and C-C only. The C-H stretching is a broad band between 2800-3000 cm-1,a band present in virtually all organic compounds In this example,the importance lies in what is not seen,i.e., the lack of bands indicates the presence of no other functional group. Chapter 12 17
Chapter 12 17 IR Spectrum of Alkanes • An alkane will show stretching and bending frequencies for C—H and C—C only. • The C—H stretching is a broad band between 2800–3000 cm-1 , a band present in virtually all organic compounds. • In this example, the importance lies in what is not seen, i.e., the lack of bands indicates the presence of no other functional group
IR Spectrum of Alkenes wavelength (um) 35 5.5 10 1213141516 80 CH(CH)CH=CH> 60 3080 (b)1-hexene C一H 40 stretch alkane stretch 1642 C=C stretch C-H bending 油围 4000 3500 3000 2500 200018001600 140012001000 800 600 wavenumber(cm-) The most important absorptions in the 1-hexene are the C=C stretch at 1642 cm-1,and the unsaturated stretch at 3080 cm-1. Notice that the bands of the alkane are present in the alkene. Chapter 12 18
Chapter 12 18 IR Spectrum of Alkenes • The most important absorptions in the 1-hexene are the C═C stretch at 1642 cm-1 , and the unsaturated stretch at 3080 cm-1 . • Notice that the bands of the alkane are present in the alkene