Organic Chemistry

1
Organic Chemistry

• William H. Brown
• Christopher S. Foote
• Brent L. Iverson

2
AlkenesStructure andNomenclature
Chapter 5
3
Unsaturated Hydrocarbons

• Unsaturated hydrocarbon contains one or more
  carbon-carbon double or triple bonds
• Alkene contains a carbon-carbon double bond and
  has the general formula CnH2n
• Alkyne contains a carbon-carbon triple bond and
  has the general formula CnH2n-2

4
Unsaturated Hydrocarbons

• Arenes benzene and its derivatives (Ch 21-22)
• we do not study arenes until Chapters 21 22
• however, we show structural formulas of compounds
  containing the phenyl group before that time
• the phenyl group is not reactive under any of
  the conditions we describe in Ch 6-20

5
Structure of Alkenes

• A double bond consists of
• one sigma bond formed by the overlap of sp2
  hybrid orbitals and one pi bond formed by the
  overlap of parallel 2p orbitals
• the two carbon atoms of a double bond and the
  four atoms bonded to them lie in a plane, with
  bond angles of approximately 120

6
Structure of Alkenes

• it takes approximately 264 kJ (63 kcal)/mol to
  break the pi bond in ethylene that is, to rotate
  one carbon by 90 with respect to the other so
  that there is no overlap between 2p orbitals on
  adjacent carbons

7
Cis,Trans Isomerism in Alkenes

• Cis,trans isomers isomers that have the same
  connectivity but a different arrangement of their
  atoms in space due to the presence of either a
  ring or a carbon-carbon double bond

8
Index of Hydrogen Deficiency

• Index of hydrogen deficiency (IHD) the sum of
  the number of rings and pi bonds in a molecule
• To determine IHD, compare the number of hydrogens
  in an unknown compound with the number in a
  reference hydrocarbon of the same number of
  carbons and with no rings or pi bonds
• the molecular formula of the reference
  hydrocarbon is CnH2n2

9
Index of Hydrogen Deficiency

• for each atom of a Group 7 element (F, Cl, Br,
  I), add one H
• no correction is necessary for the addition of
  atoms of Group 6 elements (O,S) to the reference
  hydrocarbon
• for each atom of a Group 5 element (N, P),
  subtract one hydrogen

IDH
2
10
Index of Hydrogen Deficiency

• Problem isopentyl acetate has a molecular
  formula of C7H14O2. Calculate its IHD
• reference hydrocarbon C7H16
• IHD (16-14)/2 1
• Problem calculate the IHD for niacin,
  molecular formula C6H6N2O
• reference hydrocarbon C6H16
• IHD (16 - 6)/2 5

O
O
Isopentyl acetate
11
IUPAC Nomenclature

• 1. Number the longest chain of carbon atoms that
  contains the double bond in the direction that
  gives the carbons of the double bond the lowest
  numbers
• 2. Locate the double bond by the number of its
  first carbon
• 3. Name substituents
• 4. Number the carbon, locate and name
  substituents, locate the double bond, and name
  the main chain

12
Common Names

• Despite the precision and universal acceptance of
  IUPAC nomenclature, some alkenes, particularly
  low-molecular-weight ones, are known almost
  exclusively by their common names

13
Common Names

• the common names methylene, vinyl, and allyl are
  often used to show the presence of the following
  alkenyl groups

14
The Cis,Trans System

• Configuration is determined by the orientation of
  atoms of the main chain

15
The E,Z System

• uses priority rules (Chapter 3)
• if groups of higher priority are on the same
  side, the configuration is Z (German, zusammen)
• if groups of higher priority are on opposite
  sides, the configuration is E (German, entgegen)

16
The E,Z System

• Example name each alkene and specify its
  configuration by the E,Z system

17
Cis,Trans Isomerism

• Cycloalkenes
• in small-ring cycloalkenes, the configuration of
  the double bond is cis
• these rings are not large enough to accommodate a
  trans double bond

18
Cis,Trans Isomerism

• trans-cyclooctene is the smallest trans
  cyclooctene that has been prepared in pure form
  and is stable at room temperature
• the cis isomer is 38 kJ (9.1 kcal)/mol more
  stable than the trans isomer
• the trans isomer is chiral even though it has no
  chiral center

19
Dienes, Trienes, and Polyenes

• For alkenes containing two or more double bonds,
  change the infix -en- to -adien-, -atrien-, etc.
• those containing several double bonds are often
  referred more generally as polyenes
• following are three dienes

20
Dienes, Trienes, and Polyenes

• for alkenes with n double bonds, each of which
  can show cis,trans isomerism, 2n stereoisomers
  are possible
• example 22 4 cis,trans isomers are possible
  for 2,4-heptadiene

21
Dienes, Trienes, and Polyenes

• vitamin A, a biologically important compound for
  which a number of cis,trans isomers is possible
• there are four double bonds about which cis,trans
  isomerism is possible, for 24 16 stereoisomers

22
Physical Properties

• Alkenes are nonpolar compounds
• The only attractive forces between their
  molecules are dispersion forces
• The physical properties of alkenes are similar to
  those of alkanes

23
Terpenes

• Terpene a compound whose carbon skeleton can be
  divided into two or more units identical with the
  carbon skeleton of isoprene

24
Terpenes

• Myrcene, C10H16, a component of bayberry wax and
  oils of bay and verbena
• Menthol, from peppermint

25
Terpenes

• ?-Pinene, from turpentine
• camphor, from the camphor tree

26
Fatty Acids

• Animal fats and vegetable oils are both triesters
  of glycerol, hence the name triglyceride
• hydrolysis of a triglyceride in aqueous base
  followed by acidification gives glycerol and
  three fatty acids
• fatty acids with no CC double bonds are called
  saturated fatty acid
• those with one or more CC double bonds are
  called unsaturated fatty acids

27
Fatty Acids

• the most common fatty acids have an even number
  of carbons, and between 12 and 20 carbons in an
  unbranched chain
• the CC double bonds in almost all naturally
  occurring fatty acids have a cis configuration
• the greater degree of unsaturation, the lower the
  melting point
• triglycerides rich in unsaturated fatty acids are
  generally liquid at room temperature and are
  called oils
• triglycerides rich in saturated fatty acids are
  generally semisolids or solids at room
  temperature and are called fats

28
Fatty Acids

• the four most abundant fatty acids

29
Fatty Acids

• carbon chains of saturated fatty acids exist
  largely in the staggered, anti-conformation
• because of their high degree of order, they pack
  together well and are held together by dispersion
  forces
• as a result both saturated fatty acids and
  triglycerides derived from them are solids at
  room temperature
• following is a saturated triglyceride

30
Fatty Acids

• cis double bonds place kinks in the chains of
  unsaturated fatty acids
• unsaturated fatty acids and the triglycerides
  derived from them do not pack as well in a
  crystal lattice as their saturated counterparts,
  and have weaker dispersion forces between their
  molecules
• butter fat, for example, has a high content of
  saturated fatty acids and is a solid at room
  temperature
• salad oils (from plant oils) have a high content
  of polyunsaturated fatty acids and are liquid at
  room temperature

31
Alkenes Structure andNomenclature
End Chapter 5