Chapter 2

Introduction to organic compounds

NomenclaturePhysical propertiesConformation

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

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

HH 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 CH3H3C

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 nameperferred

Ch 2 #10

commonly used alkyl groups

NH2 sec-butylamine

OH

isobutyl alcohol

Ch 2 #11

(Systematic) nomenclature of alkanes1. 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 whenused 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

systematic1. 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

HH

H

H

H

C

C

H

H

HH

H HH

H

HHH

CH2

CH

CH2

C

CH3

CH3 CH3H3C

H3C

C C

CC

C

H HH

H

HH H

H

H

OC

H

H

H

O

OCH3

OCH3

OH OH

O

Ch 2 #18

Cycloalkanes cycloalkane ~ cyclic alkane ~ alkane in a ring, CnH2n acyclic ~ open-chain

Nomenclature1. (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

nomenclaturealkyl 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

alkoxyalkanehaloalkane

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 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 #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 RNH2 all sp3 C, 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] andEN 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

OHOH

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 conformerstaggered 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 gaucheeclipsedgauche 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 < 3

equivalent 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 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

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

CH3H

H

1

3

5 123

Ch 2 #51

HH

MeMe

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’

CH2CH3H

H

HH CH3

CH3

Ch 2 #52

MeMe

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

MeMe

Ch 2 #53

trans-1,2-Me2cx 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-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

Ch 2 #55

Fused rings trans-fused rings are more stable.

hormones, steroids, cholesterol

Ch 2 #56

Chapter 2��Introduction to �organic compoundsOrganic compoundsAlkanes슬라이드 번호 4Constitutional isomersConstitutional isomers in alkanes슬라이드 번호 7Alkyl substituents [groups]Isomeric alkyls슬라이드 번호 10슬라이드 번호 11(Systematic) nomenclature of alkanes슬라이드 번호 13슬라이드 번호 14슬라이드 번호 15슬라이드 번호 16슬라이드 번호 17Skeletal structureCycloalkanes슬라이드 번호 20Alkyl halidesEthersAlcoholsFunctional group슬라이드 번호 25슬라이드 번호 26Amines슬라이드 번호 28슬라이드 번호 29Structure of RX, ROR’, ROH, and RNH2Intermolecular interactions [forces]슬라이드 번호 32Physical properties of RY슬라이드 번호 34슬라이드 번호 35슬라이드 번호 36Conformation and configurationConformations of ethane슬라이드 번호 39슬라이드 번호 40Conformations of butane슬라이드 번호 42슬라이드 번호 43Conformations of cycloalkanes슬라이드 번호 45슬라이드 번호 46슬라이드 번호 47Drawing cx (chair)Conformations of cxRing flip of cxMonosubstituted cx슬라이드 번호 52Disubstituted cx슬라이드 번호 54슬라이드 번호 55Fused rings