Organic Chemistry 9th Edition

Published by Brooks Cole
ISBN 10: 1305080483
ISBN 13: 978-1-30508-048-5

Chapter 7 - Alkenes: Structure and Reactivity - Problem: 11

Answer

(a) $-CH_{3}$ has a higher priority than $-H$ (b) $-Cl$ has a higher priority than $-CH_{2}Cl$ (c) $-CH=CH_{2}$ has a higher priority than $-CH_{2}CH_{2}Br$ (d) $-OCH_{3}$ has a higher priority than $-NHCH_{3}$ (e) $-CH=O$ has a higher priority than $-CH_{2}OH$ (f) $ -CH=O$ has a higher priority than $-CH_{2}OCH_{3}$

Work Step by Step

According to the Cahn–Ingold–Prelog priority rules : 1. An atom with higher atomic number ranks higher than the one with a lower atomic number. 2. If no difference is found (if the atoms have the same atomic number), the next atom is considered, and this is continued until the difference is found. 3. For multiple bonded atoms, consider them bonded to the same number of single bonded atoms. (a) $-CH_{3}$ has a higher priority than $-H$ According to rule 1, C has a higher atomic number than H, and thus gets higher priority. (b) $-Cl$ has a higher priority than $-CH_{2}Cl$ According to rule 1, Cl has a higher atomic number than C, and is thus assigned higher priority. (c) $-CH=CH_{2}$ has a higher priority than $-CH_{2}CH_{2}Br$ According to rule 3, a double bond corresponds to the same number of single bonded atoms attached to the atom, i.e. for $-CH=CH_{2}$, C is bonded to one H and 2 C atoms (CCH), whereas in $-CH_{2}CH_{2}Br$, the C is bonded to one C and 2 H atoms (CHH). Hence, higher priority is given to $-CH=CH_{2}$ as the atomic number of C is greater than H. (d) $-OCH_{3}$ has a higher priority than $-NHCH_{3}$ According to rule 1, O has a higher atomic number than N and thus $-OCH_{3}$ gets a higher priority. (e) $-CH=O$ has a higher priority than $-CH_{2}OH$ According to rule 3, in $-CH=O$, the C atom is bonded to one H and two O atoms (OOH) whereas in $-CH_{2}OH$, the C atom is bonded to 2 H atoms and one O atom (OHH). (f) $ -CH=O$ has a higher priority than $-CH_{2}OCH_{3}$ According to rule 3, the carbon in $ -CH=O$ is bonded to one H and 2 O atoms (OOH) and the carbon in $-CH_{2}OCH_{3}$ is attached to 2 H and one O atom (OHH).
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