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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

<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

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

CH3

HH

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’

CH2CH3

HH

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