Alkanes_Alkenes_Alkynes_Aromatic

2.4.1 ALKANESThe spectra of normal alkanes can be interpreted in terms of four vibrations:C-H and C-C bonds stretching (ν) and bending vibrations (δ)ν C-C 1200-800 cm-1weak, they are generally of little value for identificationδ C-C below 500 cm-1, do not appear in our spectraThe most characteristic vibrations are those arising fromC-H stretching and bending.ν C-H 3000-2840 cm-1CH3νas 2962 cm-1νs 2872 cm-1CH2νas 2926 cm-1νs 2853 cm-1CH3δas 1450 cm-1δs 1375 cm-1CH2δsc 1465 cm-1δrock 720 cm-1δtwist/wag 1350-1150 cm-1Branched-chain alkanesThe changes in skeletal stretching vibrations and methyl bending vibrations (δ CH3 ), these occur below 1500 cm-1tertiary –C-H ~ν 2890 cm-1gem-dimethyl doublets δ1385-1380 cm-11370-1365 cm-1Cyclic alkanesIncreasing ring strain moves the C-H stretching bands progressively to high frequencies. The ring CH2, CH groups in a monoalkylcyclopropane ring absorb in region of 3100 –2990 cm-1 (ν CH2).Cyclization decreases the frequency of the CH2 scissoring vibrations δsc CH2. cyclohexane absorbs at 1452 cm-1 (n-hexane 1468 cm-1)cyclopropane 1442 cm-1ALKENESAlkene structures introduce several new modes of vibration into a hydrocarbon molecule: ν C=C stretching vibrationsν C-H stretching vibrations in which the carbon atom is present in the alkene linkageδ =C-H in-plane and out-of-plane bending of the alkene C-H bondUnconjugated linear alkenesν C=C 1667-1640 cm-1monosubstituted alkenes 1640cm-1disubstituted trans-alkenes 1670 cm-1The absorption of symmetrical disubstituted trans-alkenes or tetrasubstituted alkenes may be extremely weak or absent.The cis-alkenes absorb more strongly than trans-alkenes.Internal double bonds generally absorb more weakly than terminal double bonds because of pseudosymmetry.Conjugated alkenes ν C=CThe spectrum of an unsymmetrical conjugated diene shows absorption near 1650 and 1600 cm-1.Conjugation of an alkene double bond with an aromatic ring produces enhanced alkene absorption near 1625 cm-1The absorption frequency of the alkene bond in conjugation with a carbonyl group is lowered by about 30 cm-1, the intensity of absorption is increased.Cumulated alkenesνas C=C=CH2near 2000-1900 cm-1ν =C-H > 3000 cm-1In general, any C-H stretching bands above 3000 cm-1 result from aromatic, heteroaromatic, alkyne, or alkene C-H stretching.Also found in the same region are the C-H stretching in small rings (such as cyclopropane) and the C-H in halogenated alkyl groups.δoop =C-H 1000-650 cm-1The most characteristic vibrational modes of alkenes are the out-of-plane C-H bending vibrations between 1000 and 650 cm-1.These bands are usually the strongest in the spectra of alkenes.In allene structures , strong absorption is observed near 850 cm-1, arising from =CH2 wagging.CycloalkenesAbsorption of the internal double bond in the unstrained cyclohexene system is essentially the same as that of a cis isomer in an acyclic system.The C=C stretch vibration is coupled with the C-Cstretching of the adjacent bonds. As the angle between C=C and C-C bond becomes smaller the interaction becomes less until it is at 90 in cyclobutene (1566 cm-1). In the cyclopropene structure, interaction again becomes appreciable, and the absorption frequency increases (1641 cm-1).The substitution of alkyl groups for an α-hydrogen atom in strained ring systems serves to increase the frequency of C=C absorption. (1-methylcyclobutene absorbs at 1641 cm-1).The absorption frequency of external exocyclic bonds increases with decreasing ring size.ALKYNESThe two stretching vibrations in alkynes involveν C≡C and ν C-H stretching.ν C≡C2260-2100 cm-1weak bandBecause of symmetry, no C≡C band is observed in the IR for symmetrically substit uted alkynes.In the IR spectra of monosubstituted alkynes, the band appears atν -C≡CH2140-2100 cm-1of disubstituted alkynesν -C≡C- 2260-2190 cm-1The intensity of the ν C≡C stretching band is increased by conjugation with a carbonyl group.ν≡C-H3333-3267 cm-1 monosubstituted alkynesThis is a strong band and is narrower than the hydrogen-bonded OH and NH bands occurring in the same region.C-H bending vibrationsδ≡C-H700-610 cm-1strong and broad absorptionThe first overtone of the C-H bending vibration appears as a weak broad band in the 1370-1220 cm-1 region.2.4.2 AROMATIC COMPOUNDSmononuclearν =C-H 3100-3000 cm-1aromatic C-H stretching bands >3000 cm-1 2000-1650 cm-1 Weak combination and overtone bands appear in the 2000-1650 cm-1region. The pattern of the overtone bands is characteristic of the substitution pattern of the ring.ν C=C1600-1585 cm-1, 1500-1400 cm-1Skeletal vibrations, involving C=C stretching within the ring, frequently appear as doublets, depending on the nature of the ring substituents.δtp =C-H 1300-1000 cm-1 in-plane bending of the ring C-H bonds,δmtp =C-H900-675 cm-1out-of-plane C-H bending vibrations, strongly coupled to adjacent hydrogen atoms, and therefore characteristic of the number of adjacent hydrogen atoms on the ring. The bands are frequently intense and may be used for the quantitative determination of the relative concentrations of isomers in mixtures. Polynuclear aromatic hydrocarbonsLike the mononuclear aromatics, show characteristic absorption in three regions of the spectrum.ν =C-Hν C=CThe aromatic C-H stretching and the skeletal vibrations absorb in the same regions as observed for the mononuclear aromatics.δmtp =C-H the most characteristic absorption of polynuclear aromatics results from C-H out-of- plane bending in the 900 – 675 cm-1 region.These bands can be correlated with the number of adjacent hydrogen atoms on the rings.For example:Most β-substituted naphthalenes show three absorption bands due to out-of-plane C-H bending in the 900-675cm-1 region.1) an isolated alfa H atom 862-835 cm-12) 2 H atoms on one ring 835-805 cm-13) 4 H atoms on the other ring 760-735 cm-1In the spectra ofα-substituted naphthalenes the bands for the isolated H and the two adjacent H atoms of β-naphthalenes are replaced by a band for three adjacent H atoms. This band is near 810-785 cm-1.The general appearance of monosubstituted benzenes and ortho, meta and para disubstituted benzenes in the infrared spectra. Some intensity variations and some frequency shifts may be expected with different substituents.。