CHE 211 Study Guide - Midterm Guide: Carbon-12, Chemical Formula, Human Development Index
Organic Chemistry 2: Infrared Spectroscopy
Functional Groups
Hydrogen Deficiency Index
o HDI: is the Hydrogen Deficiency Index or degree of unsaturation
o The degree of unsaturation presents useful information on how the atoms of the
molecule are connected or put together
o The HDI tells you how many degrees of unsaturation a particular molecule contains
(degrees of unsaturation contains double bonds, triple bonds and rings); a double bond
have 1 degree of unsaturation, a triple bond contains 2 degrees of unsaturation and a
ring contains 1 degree of unsaturation/0
o The degree of unsaturation can be found using the following equation and the
molecular formula:
o #Carbon - 1/2 # Hydrogen - 1/2 # Halogens + 1/2 # Nitrogens + 1
o Example: Determine the degree of unsaturation of a compound with the molecular
formula C8H8NOBr
▪ Answer = 8 - 1/2 (8) - 1/2 (1) + 1/2 (1) +1 = 5
▪ Therefore the structure of this compound may contain any combination of rings
ad or π ods that add up to 5
▪ Anytime you have a degree of unsaturation greater than or equal to 4, this
usually implies that the compound contains an aromatic ring (most likely
benzene ring)
▪ The ring itself would account for 1 degree of unsaturation and since there are 3
pi bonds within that ring, this makes up the other 3 degrees of unsaturation for a
total of 4 (this is for benzene)
Characteristics Patterns of Functional Groups
What to look for in an IR spectrum:
▪ Shape of the peaks
▪ Intensity of the peaks
▪ Both the shape and intensity of the peaks are unique to a particular bond and
add to the structural information we can gain from the spectra
▪ Where things absorb on the spectra is just as important as what they look like
find more resources at oneclass.com
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Document Summary
What to look for in an ir spectrum: shape of the peaks, both the shape and intensity of the peaks are unique to a particular bond and. All of the absorptions are due to c-h stretching and bending. Alkane c-h stretch show a medium to strong peak in the range of 2970-2800 cm-1. C-h bending appears also as a strong to medium peak in the region of 1485-1445 cm-1. The spectrum of is seen on the pic (left) Alkenes show several characteristic stretching absorptions that can be used for. =c-h stretch is a variable peak that appears to the left of 3000cm-1, usually around 3020- The c=c stretch usually appears in the range of 1620-1680 cm-1. Alkenes show a variety of bending absorptions that depend on the substituted pattern of the alkene which can be seen in the table below: e(cid:454)hi(cid:271)it a c c stret(cid:272)hi(cid:374)g a(cid:271)sorptio(cid:374) i(cid:374) the ra(cid:374)ge of (cid:1006)(cid:1005)(cid:1004)(cid:1004)- The spectrum of alkyl halides are similar to alkanes.
