Chapter 2

Introduction to organic compounds

Nomenclature Physical properties Conformation Organic compounds Ch 2 #2

 in Organic Chemistry 1

[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 ≡]

[HC] ~ contains only C and H carbohydrate

 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

 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 (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 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 [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 ~ 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 [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

 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 ?

 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 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