Chapter 2
Introduction to
organic compounds
Nomenclature
Physical properties
Conformation
Organic compounds
in Organic Chemistry 1
hydrocarbons [RH]
alkanes
alkenes
alkynes
alkyl halides [RX]
ethers [ROR’]
alcohols [ROH]
amines [RNH2]
in Org Chem 2
aromatic comp’ds
carbonyl comp’ds
Ch 2 #2
Alkanes
saturated hydrocarbons
saturated ~ all single bonds; no multiple bond [= or ≡]
hydrocarbon [HC] ~ contains only C and H
homologs
general formula ~ CnH2n+2
differs by CH2 (methylene)
paraffins
non-polar, hydrophobic
<cf> carbohydrate
Ch 2 #3
Ch 2 #4
Constitutional isomers
isomers [
異性質體]
same composition, different structure (and shape)
constitutional isomer
= structural isomer = skeletal isomer two or more compounds with
the same molecular formula [composition]
different structural formula [connectivity]
e.g. C2H6O
eg C4H10
H C C O H H
H H
H
H C O C H H
H
H
H
Ch 2 #5
Constitutional isomers in alkanes
straight-chain vs branched alkanes
neopentane
‘iso’ ~ C bonded to 1 H and 2 methyls [CH3]
Ch 2 #6
# of possible isomers as # of atoms
C20H42 has 366,319 isomers!
drawn? calculated?
nomenclature ~ naming
common name = trivial name
systematic name = IUPAC name
Ch 2 #7
Alkyl substituents [groups]
R ~ alkyl
RH is alkane, and
R with =, alkenyl; R with ≡, alkynyl
If R covers alkyl, alkenyl, and alkynyl, RH is HC.
Ch 2 #8
propyl
(n-)propyl ~ CH3CH2CH2-
isopropyl ~ (CH3)2CH-
butyl
Degree of substitution of carbon
CH2
CH
CH2
C
CH3 CH3 H3C CH3
H3C
Isomeric alkyls
n ~ normal, commonly omitted
CH3
primary [1°]
carbon
secondary [2°]
carbon tertiary [3°]
carbon quaternary [4°]
carbon
sec- (or s-) tert- or t-
Ch 2 #9
primary hydrogen?
pentyl
pentyl isopentyl tert-pentyl
sec-? sec-? neopentyl
IUPAC name perferred
Ch 2 #10
commonly used alkyl groups
NH2 sec-butylamine
OH
isobutyl alcohol
Ch 2 #11
(Systematic) nomenclature of alkanes
1. Determine the number of carbons in the longest continuous chain.
longest continuous chain = parent HC = root chain
‘root+ane’
Ch 2 #12
2. Number the chain so that the substituent gets the lowest number.
#-[substituent][parent]
no # in common name
iso, sec-, tert- are common names;
but accepted to IUPAC system when used as part of substituent
Ch 2 #13
3. Number the substituents to yield the lowest possible number.
Substituents are listed in alphabetical order.
If two or more same subs, use di, tri, tetra, penta, ---
‘di, tri, ---’ and ‘sec-, tert-’ are ignored in alphabetizing
‘iso’ and ‘cyclo’ are not ignored
Ch 2 #14
4. Assign the lowest possible numbers to all of the substituents
5. If the same numbers in both directions, the first group cited receives the lower number
Ch 2 #15
6. If two or more longest chains of the same length, the parent is the chain with the greatest number of subs.
Ch 2 #16
7. For branched substituent,
may use common name; iso, sec-, tert-
much simpler
systematic
1. Find the longest chain beginning at the branch.
2. Number from the branching point.
3. Put (#-name) in parentheses.
* ‘di, tri, ---’ are not ignored in alphabetizing.
5-(2-methylpropan-1-yl)decane
Ch 2 #17
Skeletal structure
skeletal structure = bond-line structure
draw by
drawing a line for a (C-C) bond
not showing C and H bonded to C
line(-bond) structure
= Kekule structure
C C C C C H
H
H C
C
H H H
H H
H C
C H
H H H
H H H
H
H H H
CH2
CH CH2
C
CH3 CH3 H3C CH3
H3C
C C C C C
H H H
H
H
H H H
H O
C H
H H
O
O CH3
OCH3
OH OH
O
Ch 2 #18
Cycloalkanes
cycloalkane ~ cyclic alkane ~ alkane in a ring, C
nH
2n acyclic ~ open-chain
Nomenclature
1. (subs)cycloalkane
If subs has more C than ring, cycloalkylalkane
2. Name two subs’ in alphabetical order; Give 1- to the first.
Ch 2 #19
3. If more than 2 subs’: i) List alphabetically, ii) Give 1- to the subs letting the second subs the lowest #, iii) So on.
4-ethyl-1,2-dimethylcyclohexane
Ch 2 #20
Alkyl halides
RX
types
nomenclature
alkyl halide (common) or haloalkane (IUPAC)
Ch 2 #21
Ethers
ROR (symmetrical) or ROR’ (unsymmetrical)
nomenclature
common name ~ alkyl alkyl ether
Common name is common [preferred] for simple ethers.
IUPAC name ~ alkoxyalkane
( )
Ch 2 #22
Alcohols
ROH ~ with hydroxy [OH] group
types
nomenclature
common name ~ alkyl alcohol
IUPAC name ~ alkanol
‘ol’ for hydroxy ‘functional group’
Ch 2 #23
Functional group
center of reactivity in molecules site where reaction takes place
priority of functional groups
alkoxyalkane haloalkane
Ch 2 #24
IUPAC nomenclature for comp’d with functional group
# just before ‘ol’ or before name
Find the longest chain containing functional group [FG]
Give lowest # to C with FG
diol, triol, ---
Ch 2 #25
For FG and subs, FG gets lowest #. priority of FG
If # the same for FG, then lowest # for subs
If more than 2 subs, alphabetical order
Ch 2 #26
Amines
RNH
2,RR’NH, RR’R”N
types ~ depends on # of alkyls not on DS of C
nomenclature
common name ~ alkylamine, alkylalkylamine, -- (one word)
Ch 2 #27
IUPAC name ~ alkanamine
rules the same as for alcohols
lowest # for amine; then for subs; subs alphabetical
N- for 2° and 3° amines
Ch 2 #28
quaternary ammonium salt
OH
NH2
5-aminohexan-2-ol
N triethylamine
N,N-diethylethanamine
Ch 2 #29
Structure of RX, ROR’, ROH, and RNH 2
all sp
3C, O, and N
Ch 2 #30
(1) instantaneous dipole-induced dipole interaction
betw non-polar molecules
(London) dispersion force
weak
(2) dipole-dipole interaction
betw polar molecules [permanent dipoles]
stronger than (1)
van der Waals force
usually, (1) + (2) ~ 0.5 – 5 kcal/mol
in a narrow sense, (1) only
Intermolecular interactions [forces]
Ch 2 #31(3) hydrogen bonding
dipole-dipole interaction
betw H on EN atom [N, O, F] and EN atom [N, O, F]
fairly strong (3 – 8 kcal/mol)
due to high ∆EN and
short distance (small H)
H on C? H on Cl?
strength the same?
O-H is a better H-bond donor
larger ∆EN
-N: is a better accepter
more loose e pair
H(2.1) C(2.5)
N(3.0) O(3.5) F(4.0) Cl(3.0) δ+
δ–
Ch 2 #32
Physical properties of RY
boiling point
liquid to gas ~ separation ~ depends on intermol force
bp with size [molecular weight] larger contact area
RH ~ low bp (1) only
ROR’ ~ bp higher than RH (2)
ROH ~ much higher bp (3)
amines
lower bp than ROH
relative H-bond strength
bp ~ 1° > 2° > 3°
RX
bp ~ RF < RCl < RBr < RI
larger µ larger polarizability larger X
Ch 2 #33
melting point
solid to liquid ~ mobility ~ also dep on intermol forces
trend the same to bp
except for the effect of molecular shape
symmetric, compact close packing high mp
even-odd effect p95
mp bp
Ch 2 #34
solubility
dissolution = mixing solvent [1] and solute [2]
∆Gmix = ∆Hmix – T ∆Smix
∆Smix > 0 always
As Temp up, T∆S up
∆Hmix depends on 1-2 interaction
intermolecular interaction betw 1 and 2
‘like dissolves like’
{polar, hydrophilic, water-soluble} vs
{nonpolar, hydrophobic, oil-soluble [organic]}
RH ~ nonpolar ~ water-insoluble
floats on water ~ density of C30 < 1
Ch 2 #35
ROH ~ water-solubility depends on size and shape of R
propanol soluble with water; butanol not
butyl alcohol less soluble than t-butyl alcohol
ROR’ ~ less water-soluble than ROH
Ether is a good choice of solvent for organic reactions.
not very reactive [stable], not very polar [dissolves organics]
Lewis base [dissolves salts (cations)], not protonic [useful for base]
amine ~ 1° > 2° > 3° more water-soluble
RX ~ R-F more water-soluble polarity and H-bonding
OH
OH
Ch 2 #36
Conformation and configuration
conformation
spatial arrangements formed by rotation around single bond
2 conformers ~ 1 compound ~ not separable
configuration
spatial arrangements formed with breaking (double) bond
2 isomers ~ 2 comp’ds ~ different properties ~ separable
Ch 2 #37
Conformations of ethane
Rotation around C-C bond gives 2 conformations.
conformer = conformational isomer? = rotational isomer?
= configurational isomer? ~ NOT isomer, but one compound
Staggered conformer is of lower energy.
due to hyperconjugation?
C-H σ and C-H σ*
due to (the absence of) repulsion between C-H bonding electrons ~ torsional strain ~ 1 kcal/mol x 3
eclipsed conformer staggered conformer
Ch 2 #38
Newman projection and potential energy map
Actually, numerous conformations.
3 max’s (eclipsed) and 3 min’s (staggered)
rotate C2 60°
front carbon (C1) rear carbon (C2)
dihedral angle [二面角]
Ch 2 #39
∆G = – RT ln K K = exp [– ∆G/RT]
K = exp [– 2.9/(.002)(300)] = .008 at 300 K
Prob(eclipsed) = .008/1.008 = .8% at 300 K
Most of ethane molecule is in staggered conformation.
= Ethane is in staggered conformation most of times.
RT RT
K
Ch 2 #40
Conformations of butane
3 max (syn, eclipsed) and 3 min (anti, gauche)
anti gauche
eclipsed
gauche eclipsed
(syn)
Ch 2 #41
anti
of the lowest energy (most stable)
gauche
higher energy than anti due to
steric strain ~ repulsion between (non-bonded) groups ~ 0.87
eclipsed
torsional + steric strain
1 x 3 + 0.4 x 2 = 3.8
H3C CH3
Ch 2 #42
(syn)
of the highest energy
torsional + steric strain
1 x 3 + 1.5 = 4.5
higher alkanes
all-anti planar zigzag ~ most stable, but not most probable
Ch 2 #43
Conformations of cycloalkanes
6- (and 5-)membered rings are most popular.
Cyclic comp’ds are strained. (angle+torsional+steric strain)
strain ~ 6, 12 or larger
<
5, 7-11<
4<
3equivalent to Table 2.9 p104
Ch 2 #44
cyclopropane
(has to be) planar
high angle strain
high torsional strain (planar)
most highly strained
cyclobutane
if planar, 90° bond angle and fully eclipsed
by puckering, angle strain , torsional strain
slightly nonplanar [puckered] ~ butterfly
still, (highly) strained
Ch 2 #45
cyclopentane
If planar, 108° bond angle (no angle strain) and eclipsed
puckered to relieve torsional strain
envelope
little strained
cyclohexane
If planar, 120° and fully eclipsed
puckered to reduce angle and torsional strain
chair comformation
virtually strain free (110° and staggered)
Ch 2 #46
cycloheptane
nonplanar
a little higher (angle and torsional) strain than cx, close to cyclopentane
rings betw C
8– C
11 very small angle and torsional strain
transannular [cross-ring] strain (interior of the ring) arises
similar total strain to those of C5 and C7, but not so popular
rings larger than C
12 strain-free
not popular
Ch 2 #47
Drawing cx (chair)
3 pairs of parallel ring bonds
6 axial and 6 equatorial (subs) bonds
4 5 H
H
axial hydrogen
equatorial H
H H
Ch 2 #48
Conformations of cx
chair and boat conformation
Boat conformer is of higher strain
torsional ~ 4 eclipsed
steric ~ flagpole H
Ch 2 #49
Ring flip of cx
chair – boat – chair
axial-equatorial change
low E barrier ~ rapid equili of chairs
twist-boat
Ch 2 #50
Monosubstituted cx
methylcyclohexane
2 chair conformations are not identical (in energy)
axial-Me-cx is of higher steric strain than equatorial-Me-cx.
due to 1,3-diaxial interactions
Energy of 1,3-diaxial = E of 2 gauches = 2 x .87 = 1.74 kcal/mol
CH3 CH3
CH3 H
H
1 3
5 3 2 1
Ch 2 #51
H H
Me
Me Me
Equili favored to equatorial
∆G = –1.74 kcal/mol = –RT ln K
K = exp [1.74/.6] = 18 at 300 K
Prob(equatorial) = 18/(1+18) = .95 at 300 K
CH3 CH3
K
‘frozen’
CH2CH3 H
H
H
H CH3
CH3
Ch 2 #52
Me Me
Disubstituted cx
1,2-dimethylcyclohexane
cis-trans isomers [geometric isomers]
not conformers
Each has conformers.
different configuration
need breaking bonds to change
different compounds with different mp, bp, ---
Me Me
Me Me
Ch 2 #53
trans -1,2-Me
2cx is more stable.
.87 x 3 = 2.6 kcal/mol
cis-
trans-
.87 x 4 = 3.5 kcal/mol .87 kcal/mol
Ch 2 #54
1,4-Me
2cx
trans-isomer is more stable. ~ fully explained in the textbook
1,3-Me
2cx
cis-isomer more stable ~ prove this by yourself
1- tert -butyl-3-methylcyclohexane
Ch 2 #55
Fused rings
trans-fused rings are more stable.
hormones, steroids, cholesterol
Ch 2 #56
