Organic Chemistry

You will need to buy a separate book “Organic Notes” Purchase from Mrs. Marston Why study Organic Chemistry? Organic chemistry is the chemistry generally practiced by living things. For biological efficiency, need

readily available atoms and light atoms. The chemistry is primarily based on C, H, N and O; also some P, Cl, Br, S (and some transition

elements in enzymes) At the moment ~ 1 x 107 organic compounds described in the literature either natural or unnatural Why is carbon so great? Its multiple bonding structures allow a great deal of complexity from a few

different elements. Carbon can form stable C-X, C=X, C≡X bonds, X = C, O, and N. No other element can do this.

Thus, many different structural possibilities exist. In the organic chemistry section, we are going to learn only 21 reactions and no new conceptual ideas.

Structures of Organic Compounds

Carbon is tetravalent - 4 covalent bonds. Carbon is usually positively charged 109.5o

sp3 hybridized

Polarity Bond Bond Energy (kJ/mol) very weak δδ-C-Hδδ+ 410 C-C 350 very strong δ+C-Oδ- 350 δ+C-Nδ- 300 δ+C-Clδ- 335 polar strongly polarized δ+H-Clδ- 430 δ-N-Hδ+ 400

Weak O-O 140 I-I 140

Nomenclature

Naming compounds – this is less important except for communication (it’s a pain, but less difficult than referring to “that thing with the thingy hanging off it.”

Types of Compounds

Bond Type Class E.g CH, CC Alkanes H3C-CH3 ethane CH, CC, CN Amine H3CH2CNH2 aminoethane CH, CC, CO Alcohol H3CCH2OH ethanol CH, CC, CO Ether H3CCH2OCH2CH3 ethyl ether CH, CC, CX Alkyl Halide BrCH2CH2Br X=Cl, Br, I 1,2-dibromoethane C-C, C=C, CH Alkene H2C=CH2 ethene C-C, CH, C=O Aldehyde

CH3

CH

O

acetaldehyde C-C, CH, C=O Ketone

CH3

CCH3

O

(2-propanone) C-C, C≡C, CH Alkyne HC≡CH acetylene, ethyne

C=C, CH, Aromatic

benzene

C=O, C-O, C-C, CH Carboxylic Acid

CH3

COH

O

acetic acid

Alkane (CnH2n+2) Nomenclature: Structural Isomers

Number of Carbons Root Number of Isomers 1 Meth 1 2 Eth 1 3 Prop 1

CH3

CH2

CH3

4 But 2

CH3

CH CH3

CH3

CH3

CH2

CH2

CH3

5 Pent 3 6 Hex 5 7 Hept 9 . 10 Dec 75 . 40 62,491,178,805,831

H3C - H → H3C-CH2-H → H3C-CH2-CH2 - H Take the previous analogue, replace H with CH2H to get higher homologue.

3D Structures of Molecules

Depends on the available orbitals. For bonding, first look at the atomic orbitals: First, establish the Bonding no double / triple bonds

e.g. CH4

CHH

H

H

geometry for C (and all 2nd row elements)

4 atomic orbitals 1 x s (2s) spherically symmetric

3 x p 2 px x axis

2 py y axis

2 pz z axis 2 rules i) Hund’s rule - leave isoenergetic electrons unpaired

ii) maximize electrostatic repulsion (i.e., separate electronic pairs as much as possible) The carbon in CH4 has 4 bonds (one to each H). We need to use all four atomic orbitals to make the 4

molecular orbitals (4 SIGMA σ orbitals). Mix 1 x 2s + 3 x 2p and get 4 x sp3 orbitals. In a tetrahedral compound, the 4 groups will be separated by about 109.5° - this is the normal geometry for carbon.

CHH

H

H

109.5o

Alkenes

Use ethene as an example; each carbon has 3 bonds (1 x C, 2 x H). Need 3 atomic orbitals to give 3 molecular orbitals. 2s + 2px + 2py → 3 x sp3 σ orbitals. (Note no z-coordinates, just 3 substituents in the x-y plane).

How to maximize repulsion? Separate by 120°.

C C

H

HH

H

120o

We have used 3 of the 4 electrons on carbon, the last electron is in a pz orbital - these combine, on adjacent carbons, to make a pi bond (π bond).

C CH

HH

H

These combine to give a π-bond

C CH

HH

HC C

H

HH

H

Alkynes

Same idea as above, but only 2 substituents for the σ orbitals. Only 2 atomic orbitals needed (1 x 2s, 1 x 2px) → 2 molecular orbitals 2 x sp2).

e.g., ethyne

C C HH

180o Note, there is still one electron in each of the py and pz orbitals (→ total of 2 σ bonds and 2 π bonds on

each carbon)

C CH H

These combine to give a π-bond

py

+pz

C CH H

C CH H

Other atoms participate in the same type of hybridization. See the examples below.

ON

N

O

H

H

H

sp2

sp3

sp3

sp

sp2

More Alkane Nomenclature

Rules for naming compound 1) take longest linear chain (this gives “root”) 2) number the molecule from one end to get lowest substitution numbers 3) name and number substituents 4) if more than one substitutent, use di, tri, tetra - 5) arrange substituents in alphabetical order (excluding prefixes such as di, tri, i.e., triethyl

precedes methyl) Nomenclature other functional groups The groups that are arranged in alphabetical order are: halogens (Cl chloro, Br bromo, I iodo), NH2

amino, CN cyano, NO2 nitro, and alkyl groups: methyl CH3 (Me), ethyl CH3CH2 (Et), propyl (Pr)

CH3CH2CH2, isopropyl (iPr) CH3

CCH3

H

Rotational isomers

Bond rotation along σ-bonds takes place readily at room temperature. However, not all “twisted” structures are of equal energy. Generally, the most stable structures have big groups as far away from each other as possible. The repulsion of groups are called van der Waals interactions.

Let’s look first at a simple structure – ethane.

HHH

H H

H

H H HH

HH

60o0o

Staggered Eclipsed

H

HHHH

HYour eye

HH

HH

HH

When there are more groups, the situation is a little more complex

CH3

HH

H H

CH3

H H HH

CH3H3C 180o

0o

HHH

H CH3

CH3

H H CH3

H

CH3H60o

120o

Eclipsed Staggered Eclipsed Staggered

GAUCHE ANTI

H

CH3CH3HH

H

Your eye CH3

CH3

HH

HH

CH3

CH3HHH

H

HCH3

CH3H

HH

Other functional groups take a different priority (highest priority at top, lowest at bottom and then the

“alphabetical groups” after that.

1 Carboxylic Acid

CH3

C1

OH

O

Always C1 ethan “oic acid” (acetic acid)

2 Ketone

CH3

CCH3

O

2-propan “one” (acetone)

3 Aldehyde

CH3

C1

H

O

propan “al” (1-propanal)

4 Alkene H3CHC=CH2 1-prop “ene” (double bond starts at carbon 1)

5 Alkyne H3CC≡CH 1-propyne (triple bond starts at carbon 1)

1) lowest # most important - find longest chain, arrange substituents in alphabetical order and

lowest substituent # e.g.

CH3

CH C

H CH3

CH3

Cl

longest chain = 4 = butane

2) if same numbers, irrespective of which end you begin with, choose alphabetical and lower

numbers,

i.e., 2-chloro-3-methylbutane not hendecagon

3-chloro-2-methylbutane

∴ CH3

CH C

H2

C CH2

CH3

BrCl

CH3 4-bromo-2-chloro-4-methylhexane is correct 3-bromo-5-chloro-3-methylhexane is not: the lowest number is higher in this name

Order of preference for naming

Highest

RCOOH > RCHO >

RR

O > R3C-OH > RNH2 > R2C=CR'2 > RC≡≡ CR' carboxylic acid aldehyde ketone alcohol amine alkene alkyne* then come other substituents: halo, methyl, nitro, etc. * note that this order is opposite in some books

Other Functional Groups

ETHER CH3CH2OCH3 ethyl methyl ether AMINE CH3NHCH2CH3 ethylmethylamine

(CH3)3N trimethylamine NITRO NO2

CH3CH2NO2 nitroethane (1-assumed)

NITRILE C≡N CH3CN ethanitrile

ESTER

O

CH3 OR

CH3CO2CH3 methyl ethanoate

Cyclic Alkanes (CnH2n)

Just add cyclo to name.

cyclopropane cyclobutane cyclopentane cyclohexane Substitutents on cyclic systems – geometric isomers.

CH3 CH3

CH CHCH2

CH2

CH2

CH3 CH3 CH3

CH3

CH CH

CH2

CH2

CH2

CH3

CH3

cis-1,2-dimethylcyclopentane trans-1,2-dimethylcyclopentane SAME SIDE of plane defined by ring OPPOSITE SIDES

Alkenes

ANE → ENE Naming ends is ene ethane → ethene

H3C-CH3 H2C=CH2

CH2

CH2

CH

CH

CH2

CH3

3 - heptene longest chain with double bond in it - gives double bond lowest possible number i.e., 3-pentene not 4-pentene

NB bond strength of double bond (~267 kcal/mol) less than bond strength of single bond (350 kcal/mol)

Therefore C=C is more reactive than C-C. However, for a given structural isomer, there may be two geometric isomers that are not interconvertible @ RT (you would have to break the bond to interconvert them).

GEOMETRIC ISOMERS If the groups with highest atomic number are on the same side - Z-isomer (zusammen), if on opposite

sides – E-isomer (entgegen)

CH

CH

CH3CH3

CH

CH

CH3

CH3

Z-isomer E-isomer If 2 identical atoms go to next atom in chain; next structure is Z-1-bromo-2-pentene (put starting carbon

of alkene at lowest number of chain)

CH

CH

CH

2CH

2CH3Br

Cyclic Alkenes

cyclobutene 4-methylcyclopentane

Triple bond – Alkyne

ANE → YNE

CH3 C C CH

CH3

Cl Cl

4-chloro-2-pentyne yne ending

Alcohol

ANE → ANOL

OHBr

CH3

CH C

H2

CH3

OHBr

4-bromo-2-pentanol

Ketone

ANE → ONE (“OWN”)

Cl

O ONE has precedence over other groups listed above 3-chloro-2-butanone

Aldehyde

ANE → ANAL

4-methylpentanal

O

H

Carboxylic Acids

ANE → ANOIC ACID

ETHANOIC ACID (acetic acid) 3-BROMO-PROPANOIC ACID

OH

O

OH

O

Br

Summary of Formulas & Isomers

1. Molecular Formula C4H8 C3H8 These are clearly different 2. Structural isomers: Same molecular formula – different arrangement of groups, Stereoisomers

have different properties i.e., boiling point, melting point, etc. e.g., C4H9Cl 2-chlorobutane 1-chlorobutane

Cl

Cl

Cl

Cl 2-chloro-2-methylpropane

1-chloro-2-methylpropane 3. Stereoisomers – geometric isomers Cyclic alkanes Alkenes

Cl Br Cl Br

cis-1-bromo-2-chlorocyclopropane E-2-pentene trans-1-bromo-2-chlorocyclopropane E-2-pentene 4. Rotational isomers

CH3

HH

H H

CH3

H H HH

CH3H3C 180o

0o

HHH

H CH3

CH3

H H CH3

H

CH3H60o

120o

Eclipsed Staggered Eclipsed Staggered

GAUCHE ANTI

H

CH3CH3HH

H

Your eye CH3

CH3

HH

HH

CH3

CH3HHH

H

HCH3

CH3H

HH

Alkanes: Properties and Reactions

CnH2n+2 b.p. m.p. Increasing London Forces (going down table) CH4 -164 -182.5 C2H6 -88 -183 C3H8 -42 -190 C5H12 36 -130 C10H22 174 -30 Polyethylene burns 140 (C100H202) Tg~20

the chemistry of parent defines chemistry for the series Homologous series each “homologue” 1 CH2 more Natural gas = methane and some ethane, a little propane Petroleum (black gold, texas tea) Distillation gives

Natural gas C4 < 20o pet ether C5 - C6 30-60 Ligroin C7 60-90 light naptha C5-C9 Gasoline C6 - C12 85-200 Kerosene C12-C15 200-300 Loading oil C15-C18 300-400 oil general paraffin nor. Asphalt C16 - C20 >400

Alkanes – other natural sources - “fart” produced in anaerobic bacterial decomposition (e.g., cow

stomach (blue angels)) also found in salt mines, coal mines

Reactions of Alkanes

As we already saw, not very reactive 1) strong bonds C-C, C-H 2) not polar C - C no polarization C - H small polarization

hard to start reactions. Reactions generally happen at “functional group (C=O, C-N, C≡C, etc.) Alkanes - cyclo alkanes “paraffin” means unreactive

No reactionH2SO4

No reactionKMnO4

No reactionNa

To decompose Na use alcohol in parafin – only the alcohol reacts 2Na + 2 CH3CH2OH → 2 CH3CH2ONa+ + H2

1 Halogenation with Cl2, or Br2

H

H Br2

H

Br + BrH

This is a radical chain reaction – doesn’t work in the dark

3 parts Initiation, Propagation, Termination

H

H

Br2

H

Br

+ BrH

hν Br2

BrH

H + Br2 + Br

Initiation

Propagation

Termination

Br2Br2

H + H

HBr+

H

Br

STEP 1

STEP 2

endothermic

endothermic

exothermic

net reaction exothermic

Let’s look at a simpler system CH4 + Cl2 → CH3Cl + HCl ∆Hrxn = -104 kJ mol-1 1) Cl2 → 2 Cl• ∆H1 = +243 kJ mol-1

2) Cl• + CH4 → CH3

• + HCl ∆H2 = +4 kJ mol-1 3) CH3

• + Cl2 → CH3Cl + Cl• ∆H3 = -108 kJ mol-1 But for bromination CH4 + Br2 → CH3Br + HBr ∆Hrxn = -34 kJ mol-1 1) Br2 → 2 Br• ∆H1 = +192 kJ mol-1

2) Br• + CH4 → CH3

• + HBr ∆H2 = +66 kJ mol-1 3) CH3

• + Br2 → CH3Br + Br• ∆H3 = -100 kJ mol-1

Note that chlorination is mildly endothermic in the first step of propagation (step 2) whereas bromination

is quite endothermic. In the second step of the propagation, both are exothermic. The overall reaction rate is dependent upon the activation energy in the slowest step (step 2). The Ea for chlorination is much lower that for bromination, which one might predict from the overall enthalpies of the steps.

The overall reaction with Cl2 faster than Br2 If excess halogen, e.g., Cl2, more chlorination ie. CH4 → CH3Cl → CH2Cl2 But for I2 step 1 very endothermic + 200 kJ/mol ∴ reaction very slow so not useful For F2, steps 1 and 2 are very exothermic step a ~ -144 kJ/mol → explosive reaction ∴ use other reagents to make F- alkanes (CoF3, SF4) If one uses unsymmetical alkanes, there are different types of CH bonds. Depending on which bond

reacts, different products are formed. The preference depends on the strength of the C-H bonds and on the number of hydrogens of a given type. In general, effects from both factors are observed.

The bond strengths of CH bonds depend on the number of carbons connected to the central carbon.

Reactant Products Bond Dissociation kJ mol-1

H3CH H3C• 426 H3CCH2H (Bold carbon is primary) H3CC•H2 a primary radical 405 (H3C)2CHH (Bold carbon is secondary) (H3C)2C

•H a secondary radical 397 (H3C)3CH (Bold carbon is tertiary) (H3C)3C

• a tertiary radical 376 The ease of forming a carbon radical (and the order of highest stability) is 3° (tertiary) > 2° (secondary) > 1° (primary) > methyl Recall for chlorination (and bromination) RH + Cl• → R• + HCl endo (slow) R + Cl2 → RCl + Cl• exo What happens in a molecule with both types of hydrogens – both happen

CH3

CHCH3

CH3

CH2

CH3

k1

ClCH3

CH

2

CH2

k2Cl

Cl2CH3

CH2

CH2

Cl

CH3

CH2

CH2

ClCl2

a PRIMARY alkyl radical

a SECONDARY alkyl radical The rates are proportional not only to the bond strength of the CH bond being broken, but also on the

statistical number of hydrogens. The bond strength is the most important factor. (i.e., generally k2 > k1) Rate of reaction via 1° CH ∝ k1 [CH3CH2CH2CH3][Cl•] x fn 6 H’s Rate of reaction via 2° CH ∝ k2 [CH3CH2CH2CH3][Cl•] x fn 2 H’s Where fn is some fractional effect of arising from the statistics. The product ratio between these two products will be rather similar to k1 / k2 The actual ratio of products is 1° alkyl halide 45%, 2° alkyl halide 55%

2 Alkanes - Combustion

This is the fundamental reaction of the 20th century CnH2n+2 → (3n +1)/2 O2 → nCO2 + (n+1) H2O + heat H4C + 2 O2 → CO2 + 2 H2O + ∆ e.g., cigarette lighter C4H10 + 6.5 O2 → 4CO2 + 5H2O

Alkane ∆H combustion (kJ/mol) CH4 213 H3CCH3 373 C4H10 687

(C4H8) 656 NOTE Ring strain

C5H12 845

(C5H10)

793

4.1 A

Alkyl Halides ? Haloalkanes

Chemistry controlled by bond polarity δδ-C-Hδδ+ δ+C-Xδ- X = F high dipole moment → X = I lowest polarity ∴ alkyl halides have higher m.p. & b.p.’s than related alkanes that only have London forces CH4 b.p. -164 °C H2CCl2 b.p. 35 °C

CH3

CH2

CH2

X Lewis acids attack here:

H+, Ag+, BF3

Lewis bases (nucleophiles) attack here:

-OH, :NH3, -SH, -CN

Preparation of Alkyl Halides

1 Haloalkane Preparations; From alkanes (Review)

Seen above

HCl2

Cl

hν or ∆

3 From alcohols,

OH BrH Br

2-propanol a substitution reaction

4 Addition to alkene

Br2

very fast

Br

Br

Reactions of Alkyl Halides

5 Nucleophilic Substitution

INC- + NC+ I

Many examples of substitution reactions. (For OH- need dilute solutions, see below)

6 Elimination of HX

I

HHO-

+ + I +OH2

hot

conc.

base

7 Reactions with Group 1 or 2 metals Li / Mg

Br

O

Mg

ether

MgBrδ+

δ- δ+δ-

= Organometallic

SN2 Reactions (5)

BrNaI + I+ NaBr (ppt)

δ+δ- 2-propanone

Kinetics process of process - d[EtBr]/dt = k[EtBr]1[I-]1 Second order – bimolecular; these kinetics are observed for MeX, 1° RX (RCH2X) and 2° RX

(RR'CHX) BUT NOT FOR 3° RX (RR'R"CX)

Called SN2 “substitution nucleophilic bimolecular” Go from 1 isomer to a different isomer; i.e., Inverted stereochemistry at the reaction centre Most nucleophilic substitutions take place this way.

Br

NC-

O

NC

trans cis

Mechanism of the SN2 Reaction

C

H

BrHH

HO-

C

H

Br

H H

OH

_

C

H

OH HH

Br+

The SN1 Reaction – another substitution reaction

As we saw above, 1° and 2° alkyl halides mostly undergo SN2 reactions. For 3° alkyl halides, however, need very polar solvents and non-basic nucleophiles to observe nucleophilic substitution. However, the kinetics are different.

faster in

(CH3)3CBr + N3-

→ (CH3)3C-N3 Azide polar

solvent Rate Law (Experimental !) Rate = k[RBr]1

a first order reaction; implies the overall reaction is not an elementary step (that the rate detemining step

doesn’t need a nucleophile).

Br slow

ionization C+ + Br

C+ + N3

- N3

1

2

Why doesn’t the SN2 happen with 3° alkyl halides? Two reasons – i) Steric reasons (the space the

nucleophile needs, to attack the carbon, is occupied by other groups). The alkyl groups block backside attack. ii) the 3° cation is sufficiently stable that another reaction pathway exists.

Order of stability of carbocations (just as we found for radicals): 3° ((CH3)3C+) > 2° ((CH3)2C+H) > 1° (H3CC+H2) > H3C+ Why? The alkyl groups help stabilize the cation by donating electronic charge. Smearing out charge is

energetically favourable. One can accelerate the rate of these reactions using Lewis acids. Silver removes the chloride in tertiary

chlorides to help form the cation. This is a classic Lewis acid: Lewis base reaction,

Cl+ Ag

+

ClAg C

+Cl

Ag

ppt

Nu

Nu This is a useful chemical test for 1° 2° and 3° alkyl halides Some nucleophilic substitutions

Cl

OH2

OHClH+

alcohol

water is nucleophile and solvent

Cl

EtOH

OClH+

ether

alcohol is nucleophile and solvent

ClHO-

OH alcohol

ClNC- CN nitrile

ClRNH2 N

+ R

R H

Clammonium salt

KOHN

R

Ramine

Summary of Nucleophilic Substitution

Methyl 1° 2° 3° CH3X C-CH2X C2CHX C3CX SN2 relative rate 30 1 0.03 1 x 10-6 SN1 H3C+ never see C-CH2

+ almost never see

1-10% of SN2 rate

100%

Elimination (6)

We saw above that OH- + an alkyl halide gives an alcohol (an SN2 reaction). This is true only if COLD

DILUTE OH- (KOH, NaOH) is used. If HOT CONCENTRATED (e.g., > 3M) is used, a second order

ELIMINATION occurs to give an alkene.

Br

H

OH-

OH2 +HOT

CONC

KOH In this reaction, the HO- is acting as base, not a nucleophile. The attack of at the carbon is slower than the attack of the base at the H Rate Law; Rate = k[RX][OH

-]; called E2 (elimination bimolecular)

H

Br

HO-

R

H

H

H

bond breaking

Bond Making

R

H

HH

HH

R

Generally the more carbon groups on a double bond, the more stable it is: Saytzeff’s rule

Cl

+ NaOHheat

+

major minor Bromo-3-methylbutane 3 methyl butane

Organometallics (7)

We can completely alter the electronic distribution in a molecule by converting an alkyl halide into an organometallic compound.

H3Cδ

+-Iδ- + Mg → H3Cδ--Mgδ+-I methylmagnesium iodide

Br

+ Liether Li

LiBr+

These are carbanions (very strong bases and nucleophiles). This is one of the few reactions we learn in

year 1 from which C-C bonds can be formed. THIS IS AN IMPORTANT REACTION!

8 Reaction of Organometallics with water (the carbanion is a strong base)

MgBrδ+δ-

+ HOH

pKa 15.5

etherH

pKa ca. 50

+ HOMgBr

9 Reaction of Organometallics with ketones or aldehydes (IMPORTANT C-C bond formation

#1)

MgBr δ+ δ-+ether

OR

R

δ+δ-

ketone

RR OMgBr

H3O+

RR OH

alcohol

10 Reaction with CO2 (IMPORTANT C-C bond formation #2)

MgBr δ+ δ-+ether

OO

δ+δ-

O OMgBr

H3O+

O OH

carboxylic acid

pH ca. 2

Alkenes

Preparation of Alkenes

From Haloalkanes (6)

Br

H

CH3OK

HEAT

1-bromo-3-methylbutane 3-methyl-1-butene This is the E2 mechanism we described above

11 From alcohols

an alcohol + an dehydrating acid (H2SO4, H3PO4)

OH

+ H3PO4

catalystdistill

∆ + OH2

cyclohexanol catalysts cyclohexane b.p. 156 °C b.p. 82 °C We shall discuss this below.

Alkene Reactivity

Dominated by π bonds π bond ∴ energy (~ 267 kJ/mol strength of the double bond) more reactive than a σ -bond (310

kJ/mol); It is a Lewis base - attack by E+ on π-electrons, i.e. In plane of screen above or below π-bond

H

HH

RR

H

H

H

HH

R

H3O+

C+

H

HH

R

HX

H

HH

R

HX

NOT

C+

H

HH

R

H X

H

HH

R

H

X

These are extremely reactive almost as reactive as the metal CHECK HYPERCHEM

Fats: in the body: triglycerides

O

O

O

O

O

O

1 2 3 Oleic linoleic limolenic C12 C14 C16 C18 C18 C18 C18 beef fat 0 0 27 14 49 2 - lard 0 1 24 9 47 10 0 human 1 3 27 8 48 10 - leving 0 5 14 3 0 0 30 corn 0 1 10 3 50 34 0 olive 0 0.1 7 2 84 5 0

Other important alkenes: squalene → cholesterol; β-carotene → retinal (vision) β-selinene, celery oil myrcene, bay leaf oil α-pinene, cedar leaf oil Alkenes may exist in two different geometric isomers (see above) (Z- and E-)

Z-2-butene E-2-butene This is the source of black/white vision

OH NHBODY

Opsin

hν

N

H

BODYOH

HVitamin A

β-carotene

Alkene Reactions

12 Electrophilic additions

Remember that the order of cation stability is: 3° ((CH3)3C+) > 2° ((CH3)2C+H) > 1° (H3CC+H2) > H3C+ additions to alkenes usually proceed via the most stable cation.

HXC

+

H H

XX

X = OH, need H+ as catalyst

X = Br, Cl, I get both cis and trans addition

Bromination

This is a special case of electrophilic addition.

H

R Br2H

R

Br+

HR

Br

Br

HR

Br

Br Can also use ICl (I+ Cl

-)

Only get trans addition

H+

C+

H

OH2

H

OH

Cl

H

ClBr2

Br+

BrBr

Br +

Br

Br

ICl

Cl

I

+ Cl

I

Why do Br2 and ICl add trans but HCl adds randomly: The answer: Br and I are Lewis bases

13 Reduction

In organic chemistry, addition of H’s or removal of oxygen is “reduction”. The removal of hydrogens or addition of oxygen is “oxidation”. Conversion of C=C to HC-CH is, therefore, a reduction

D

DPt catalyst

D

H

H

D

H2

cis-addition of H2, the reaction happens at the solid surface of Pt Process used commercially to “hydrogenate” fats. Oleic acid

O

O

H m.p. 4 oC

stearic acid

m.p. 70 oCO

O

H

PtH2

Stearic acid is a saturated fat; Not so good for you. Better for your health are polyunsaturated fats; they are more easily processed.

Alcohols & Ethers

Unlike the functional groups we have seen so far, alcohols and ethers (to a lesser extent) are polar molecules. In the case of alcohols, there is strong H bonding and reasonably large dipole moments.

OH

OH

pKa ca. 16

CH3OH

ethanol:

consumptioncan lead toblindness

OH

OH

2-propanol (isopropanol)

rubbing alcoholin beer, wine, liquor

methanol

Alcohols Preparation

SN2 of alkyl halides with hydroxide (5)

I

+ HO-dilute

cold

OH

+ I

Hydration of alkenes (12)

+ OH2

H2SO4

DiluteOH2

OH

Addition of organometallics to aldehyde or ketone (8)

Br

O

Mg

ether

MgBrδ+ δ- δ+δ- O

H

OMgBr

H

H3O+

OH

H

electrophilic Nucleophilic C C CH3I + 2 Li → CH3Li + LiI

From electronegativity, the polarization (and reactivity) of a ZM bond, M = Na, Li, Z = first row

elements follows the reactivity -CH3 > -NH2 > -OH > -Cl (NOTE: This is consistent with our discussion of acidity. It’s harder to make the H3C

- than Cl

-)

Any reaction that converts one of the compounds on the left to one on the right will be thermodynamically favoured.

e.g., CH3Li + H-OH → CH3-H + LiOH

Less stable More stable CH3MgBr + H-OH → CH3-H + BrMgOH Other C-C Bond Forming Reactions

MgBr δ+ δ-+ether

OR

R

δ+δ-

ketone

RR OMgBr

H3O+

RR OH

alcohol Note: CAN’T DO THE FOLLOWING REACTION.

MgBrether

BrR

R

δ+δ-

alkyl halide

RR H+

Ethers are less polar than the alcohols (No OH’s for H-bonding) They are made in a similar fashion to alcohols

Preparation of ethers (5 SN2)

Br + CH3O- Na+

O

CH3

+ NaBr

14 Making Alkoxides (Reducing Metals)

2 OH + 2Na 2 ONa + H2 Alkoxides are strong bases (stronger than hydroxide) and also good Lewis bases or nucleophiles Recall: 2HO-H + 2K → 2HO-K + H2 ↑

Reactions of Alcohols

Preparation of alkyl halides (3)

Alcohols & halogen acids e.g. HCl, HBr, HI OH + BrH

Br + OH2

strong acid weak acid Note that the reaction doesn’t work under basic conditions

OH + BrBr + HO-

weak base strong baseNa

+

The mechanism

Step 1

OH + BrH O+

H

H + Br

Step 2 O

+

H

H

+ BrBr + OH2

SN2

Alcohol & Dehydrating Acid (11)

e.g. 80% H2SO4, or H3PO4 is required

O

H

H

O+

H

H

H

C+

H

H2SO4

HSO4- OH2+

- H2SO4

cyclopentanol Elimination

15 Alcohols + Oxidizing Agents

One can use inorganic salts to oxidize (remove H’s or introduce oxygen) onto organic molecules. eg. CrO3 or K2Cr2O7/H2SO4

Chromium IV or Na3Cr2O7/H2SO4

≡ H2CrO4 Chromic acid

or KMnO4 (MnVII)

Or “Organic Chromium Salts, like pyridine chlorochromate (PCC)

N+

H

Cr

O

OCl

O

1o Alcohol C - CH2OH (a) with PCC

OH

H

+ PCCCH2Cl2

H

O

removes this H Ethanol (acetaldehyde)

(b) with K2Cr2O7/H+ or KMnO4

OH

H

H

+ KMnO4

OH

O

H

O

KMnO4

2o Alcohol + Any oxidant

OH

H

+ Na2CrO7

H2SO4 O

2-propanol 2-propanone

3o Alcohol no reaction at normal temps, No H-COH to oxidize!

Aldehydes and Ketones

Preparation

Oxidation of 1o Alcohol (15)

OH

H

+ PCC CH2Cl2

H

O

2-methylpropanol 2-methylpropanal

Oxidation of 2o Alcohol (15)

OH

H

+ KMnO4O

3-methyl-2-butanol 3-methyl-2-butanone

Reactions of Aldehydes and Ketones

Nucleophilic Addition The carbon in C=O is electron-poor, an electrophile.

Oδ+ δ-

X Yδ+δ-

X

O Y+

OR

X

O

Y

ionic

covalent

Examples

Organometallic + Ketone (8)

aldehyde → secondary alcohol ketone → alcohol

Br

O

Mg

ether

MgBrδ+

δ- δ+δ- O

Ph

OMgBr

Ph

H3O+

OH

Ph

16 Reduction with NaBH4

O

Ph NaBH4

CH3OH O

Ph

H

17 Reduction with H2 and a Pt catalyst

NOTE: This is exactly the same as addition of H2 to an alkene

O

Ph

Pt-catalyst O

Ph

H

H2

EtOH solvent

18 Reaction with N-Compounds

e.g., hydrazine H2NNH2; 2,4-dinitrophenylhydrazine O2N NO2

NH

NH2

; hydroxylamine H2NOH

O+ H2NNH2

NNH2 + OH2

O+ H2NNH2

O

NH2

+ NH2

in EtOH

O

NH2

+ NH2

OH

NHNH2

NH

+

NH2

NNH2 + OH2

19 Oxidation - Aldehydes Occurs very easily, while can use strong oxidants such as K2Cr2O7/H+ or KMnO4 Can also use weak oxidizing agents eg. Ag

H

O

+ KMnO4OH

O

+ MnO2

Another more relevant oxidant – how to make a mirror

OH

OH

OH

OH

OOH

OH

OH

OH

OH

OHO

H

Ag+

OH

OH

OH

OH

OHO

OH

+ Ag

mirror

NOTE: Ketones + oxidants No reaction at normal temps

Carboxylic Acids

Structures: Pure glacial acetic acid Methanoic acid (formic acid)

OH

O

H OH

O

Ethanoic acid (acetic acid - 5% soln in H2O is vinegar) The Structure Type

R Y

O

Is very common R = any alkyl or aryl (aromatic) e.g.

R NH2

O

R

O

OMe R Cl

O

O

O O

Amides Esters Acid chlorides acetic anhydride

Preparation of Carboxylic Acids

Organometallic + CO2 (10)

Li +C

O

Oδ+

δ- δ-

δ+ether

O

O Li+

H3O+

pH 2

O

OH

(2-Methylpropyl)lithium

Aldehyde + most oxidants (19)

H

O+ Na2Cr2O7

H3O+ OH

O

2-phenylethanal 2-phenylethanoic acid

(Phenyl acetic acid)

1o Alcohol + Any oxidant

OH

H H

+ KMnO4 O

OH

+ MnO2

ethanol ethanoic acid (acetic acid)

20 Reaction of -COOH with an Alcohol →→ Esters

Need a strong acid catalyst

OH

O

OH

+H

+

H3O+

O

OOH2

ethanoic acid ethyl ethanoate (ethyl acetate) (this is related in mechanism to reaction 18) Reaction is a Nuc. Add followed by an elimination

(2) Acidity pKa≈ 3- 5

O

OH

+H3O+

OH2O

O

+

ethanoic acid (acetic acid) -two O atoms and therefore the O-H bond is weakened moreover, there is resonance through which the charge is dispersed.

O

O

O

O

(Boy have we already covered this) Compare with Alcohols

OH

+H3O+

OH2

O

+

Only one O to share electrons pKa ≈ 16-18

NOTE: an alkane C-H is an unbelievably weak acid, nothing to stabilize the charge CH4 → CH3

- + H3O+ ie. pKa of x 50