Chapter 2
Introduction to organic compounds
Nomenclature Physical properties Conformation Organic compounds Ch 2 #2
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 Alkanes Ch 2 #3
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 Ch 2 #4 Constitutional isomers Ch 2 #5
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. C H O 2 6 H H H H H C C O H H C O C H H H H H
eg C4H10 Constitutional isomers in alkanes Ch 2 #6
straight-chain vs branched alkanes
‘iso’ ~ C bonded to 1 H and 2 methyls [CH3]
neopentane Ch 2 #7
# of possible isomers as # of atoms
C20H42 has 366,319 isomers! drawn? calculated?
nomenclature ~ naming
common name = trivial name
systematic name = IUPAC name Alkyl substituents [groups] Ch 2 #8
R ~ alkyl R with =, alkenyl; R with ≡, alkynyl
RH is alkane, and If R covers alkyl, alkenyl, and alkynyl, RH is HC. Isomeric alkyls Ch 2 #9
propyl n ~ normal, commonly omitted (n-)propyl ~ CH3CH2CH2-
isopropyl ~ (CH3)2CH-
butyl CH3
sec- (or s-) tert- or t- Degree of substitution of carbon
CH3 H3C CH3
H3C CH C
C C CH3 primary [1°] H H carbon 2 2 secondary [2°] tertiary [3°] quaternary [4°] carbon carbon carbon Ch 2 #10
primary hydrogen?
pentyl
pentyl isopentyl tert-pentyl
IUPAC name perferred
sec-? sec-? neopentyl Ch 2 #11
commonly used alkyl groups
OH
isobutyl alcohol NH2 sec-butylamine (Systematic) nomenclature of alkanesCh 2 #12 1. Determine the number of carbons in the longest continuous chain.
longest continuous chain = parent HC = root chain
‘root+ane’ Ch 2 #13 2. Number the chain so that the substituent gets the lowest number.
#-[substituent][parent]
iso, sec-, tert- are common names; but accepted to IUPAC system when no # in common name used as part of substituent Ch 2 #14 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 #15 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 #16 6. If two or more longest chains of the same length, the parent is the chain with the greatest number of subs. Ch 2 #17 7. For branched substituent,
5-(2-methylpropan-1-yl)decane
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. Skeletal structure Ch 2 #18
skeletal structure = bond-line structure
draw by line(-bond) structure drawing a line for a (C-C) bond = Kekule structure not showing C and H bonded to C
H H H H H H
CH3 H H C H C H H3C CH3
H C C C C C C H H3C CH C
C C CH3 H2 H2 H H H H C H H H H O
H H O H H C O H C O H CH3 C C H H OH OH C C H H OCH3 H H Cycloalkanes Ch 2 #19
cycloalkane ~ cyclic alkane ~ alkane in a ring, CnH2n 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 #20 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 Alkyl halides Ch 2 #21
RX
types
nomenclature alkyl halide (common) or haloalkane (IUPAC) Ethers Ch 2 #22
ROR (symmetrical) or ROR’ (unsymmetrical)
nomenclature
common name ~ alkyl alkyl ether
( )
. Common name is common [preferred] for simple ethers.
IUPAC name ~ alkoxyalkane Alcohols Ch 2 #23
ROH ~ with hydroxy [OH] group
types
nomenclature
common name ~ alkyl alcohol
IUPAC name ~ alkanol
‘ol’ for hydroxy ‘functional group’ Functional group Ch 2 #24
center of reactivity in molecules site where reaction takes place
priority of functional groups
alkoxyalkane haloalkane Ch 2 #25
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 #26
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 Amines Ch 2 #27
RNH2, RR’NH, RR’R”N types ~ depends on # of alkyls not on DS of C
nomenclature
common name ~ alkylamine, alkylalkylamine, -- (one word) Ch 2 #28
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 #29
NH2
triethylamine N OH N,N-diethylethanamine 5-aminohexan-2-ol
quaternary ammonium salt Structure of RX, ROR’, ROH, and RNHCh2 2 #30 all sp3 C, O, and N Intermolecular interactions [forces] Ch 2 #31 (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)
usually, (1) + (2) ~ 0.5 – 5 kcal/mol
in a narrow sense, (1) only Ch 2 #32 (3) hydrogen bonding
dipole-dipole interaction δ+ betw H on EN atom [N, O, F] and δ– EN atom [N, O, F]
fairly strong (3 – 8 kcal/mol)
due to high ∆EN and H(2.1) C(2.5) short distance (small H) N(3.0) O(3.5) F(4.0) Cl(3.0) 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 Physical properties of RY Ch 2 #33
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 #34
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
mp bp
even-odd effect p95 Ch 2 #35
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 #36
ROH ~ water-solubility depends on size and shape of R . propanol soluble with water; butanol not . butyl alcohol less soluble than t-butyl alcohol
OH OH
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 Conformation and configuration Ch 2 #37
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 Conformations of ethane Ch 2 #38
Rotation around C-C bond gives 2 conformations.
staggered conformer eclipsed conformer
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 Ch 2 #39
Newman projection and potential energy map
Actually, numerous conformations.
3 max’s (eclipsed) and 3 min’s (staggered)
front carbon (C1) rear carbon (C2)
rotate C2 60°
dihedral angle [二面角] Ch 2 #40
RT
RT
K
∆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. Conformations of butane Ch 2 #41
3 max (syn, eclipsed) and 3 min (anti, gauche)
(syn) gauche eclipsed anti eclipsed gauche Ch 2 #42
anti
of the lowest energy (most stable)
gauche
H3C CH3
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 Ch 2 #43
(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 Conformations of cycloalkanes Ch 2 #44
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 < 3
equivalent to Table 2.9 p104 Ch 2 #45
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 #46
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 #47
cycloheptane
nonplanar
a little higher (angle and torsional) strain than cx, close to cyclopentane
rings betw C8 – C11 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 C12 strain-free
not popular Drawing cx (chair) Ch 2 #48
3 pairs of parallel ring bonds
6 axial and 6 equatorial (subs) bonds
H axial hydrogen
H equatorial H
H 4 H 5 Conformations of cx Ch 2 #49
chair and boat conformation
Boat conformer is of higher strain
torsional ~ 4 eclipsed
steric ~ flagpole H Ring flip of cx Ch 2 #50
chair – boat – chair
axial-equatorial change
low E barrier ~ rapid equili of chairs
twist-boat Monosubstituted cx Ch 2 #51
methylcyclohexane CH3
CH3
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
H CH3 H 5 3 1 1 2 3
Energy of 1,3-diaxial = E of 2 gauches = 2 x .87 = 1.74 kcal/mol Ch 2 #52
Equili favored to equatorial CH3 ∆G = –1.74 kcal/mol = –RT ln K K CH3 K = exp [1.74/.6] = 18 at 300 K
Prob(equatorial) = 18/(1+18) = .95 at 300 K
H CH2CH3 CH H 3 H CH3 H
Me Me H Me H ‘frozen’ Disubstituted cx Ch 2 #53
1,2-dimethylcyclohexane
e Me M Me Me
Me cis-trans isomers [geometric isomers] Me
not conformers
Each has conformers.
different configuration
need breaking bonds to change
different compounds with different mp, bp, --- Ch 2 #54
trans-1,2-Me2cx is more stable.
cis-
.87 x 3 = 2.6 kcal/mol trans-
.87 x 4 = 3.5 kcal/mol .87 kcal/mol Ch 2 #55
1,4-Me2cx trans-isomer is more stable. ~ fully explained in the textbook
1,3-Me2cx cis-isomer more stable ~ prove this by yourself
1-tert-butyl-3-methylcyclohexane Fused rings Ch 2 #56
trans-fused rings are more stable.
hormones, steroids, cholesterol