Need Book "Organic Notes" Purchase from Mrs. Marsden

Why study Organic Chemistry?

This 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 ~ 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, CºX 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 - no new conceptual ideas.

Structures of Organic Compounds

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

 

Polarity	Bond	Bond Energy (kJ/mol)
very weak	dd -C-Hdd+	410
	C-C	350
very strong	d+C-Od-	350
	d+C-N d-	300
polar	d+C-Cl d-	335
strongly polarized	d +H-Cl d-	430
	d -N-H d+	400
Weak	O-O	140
	I-I	140

 

Nomenclature

Naming compounds - 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
		acetaldehyde
C-C, CH, C=O	Ketone
		(2-propanone)
C-C, CºC, CH	Alkyne	HCºCH
		acetylene, ethyne
C=C, CH,	Aromatic
		benzene
C=O, C-O, C-C, CH	Carboxylic Acid
		acetic acid

 

Alkane Nomenclature: Structural Isomers
 

Number of Carbons	Root	Number of Isomers
1	Meth	1
2	Eth	1
3	Prop	1
4	But	2
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.

First, establish the

Bonding no double / triple bonds

e.g. CH₄ 

geometry for C (and all 2nd row elements)

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

3 x p 2 p_x x axis 

2 p_y y axis 

2 p_z z axis 

2 rules i) Hunds 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 s 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.

How to maximize repulsion? Separate by 120°.

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

e.g., ethyne 

Note, there is still one electron in each of the p_y and p_z orbitals (® total of 2 s bonds and 2 p bonds on each carbon)

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

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) 

Rotational isomers

Bond rotation along s -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.

Staggered Eclipsed

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

Other functional groups take a different priority (highest priority at top, lowest at bottom and then the "alphabetical groups" after that.
 

1	Carboxylic Acid		Always C1	ethan "oic acid" (acetic acid)
2	Ketone			2-propan "one" (acetone)
3	Aldehyde			propan "al" (1-propanal)
4	Alkene	H3CHC=CH2		1-prop "ene" (double bond starts at carbon 1)
5	Alkyne	H3CCºCH		1-propyne (triple bond starts at carbon 1)

1) having the lowest number is most important - find longest chain, arrange substituents in alphabetical order and lowest substituent #

e.g. 

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-methyl-butane not 3-chloro-2-methyl butane

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 > > R₃C-OH > RNH₂ > R₂C=CR'₂ > RCº 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

Cyclic Alkanes (C_nH_n+2)

Just add cyclo to name.

cyclopropane cyclobutane cyclopentane cyclohexane

cis-1,2-dimethylcyclopentane trans-1,2-dimethylcyclopentane

Alkenes

ANE ® ENE

Naming ends is ene ethane ® ethene

longest chain with double bond in it - gives double bond lowest possible number

i.e. 3-pentene not 4-pentene

If the groups with highest atomic number are on the same side - Z-isomer (zusammen), if on opposite sides – E-isomer (entgegen)

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)

Cyclic Alkenes

cyclobutene 4-methylcyclopentane

Triple bond – Alkyne

ANE ® YNE

4-chloro-2-pentyne yne ending

Alcohol

ANE ® ANOL

4-bromo-2-pentanol

Ketone

ANE ® ONE ("OWN")

ONE has precedence over other groups listed above

3-chloro-2-butanone

Aldehyde

ANE ® ANAL

4-methylpentanal

Carboxylic Acids

ANE ® ANOIC ACID

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

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

Cyclic alkanes                                             Alkenes

cis-1-bromo-2-chlorocyclopropane E-2-pentene

trans-1-bromo-2-chlorocyclopropane E-2-pentene

4. Rotational isomers

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 -> 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

Alkanes - cycloalkanes "paraffin" means unreactive

To decompose Na use alcohol in parafin – only the alcohol reacts

2Na + 2 CH₃CH₂OH ® 2 CH₃CH₂ONa+ + H₂

1 Halogenation with Cl₂, or Br₂

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

3 parts Initiation, Propagation, Termination

Let’s look at a simpler system

CH₄ + Cl₂ ® CH₃Cl + HCl D H_rxn = -104 kJ mol^-1

1) Cl₂ ® 2 Cl·D H₁ = +243 kJ mol^-1

2) Cl· + CH₄ ® CH_3· + HCl D H₂ = +4 kJ mol^-1

3) CH_3· + Cl₂® CH₃Cl + Cl·D H₃ = -108 kJ mol^-1

But for bromination

CH₄ + Br₂ ® CH₃Br + HBr                D H_rxn = -34 kJ mol^-1

1) Br₂ ® 2 Br·                                   D H₁ = +192 kJ mol^-1

2) Br· + CH₄ ® CH_3· + HBr             D H₂ = +66 kJ mol^-1

3) CH_3· + Br₂® CH₃Br + Br·            D 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 E_a for chlorination is much lower that for bromination, which one might predict from the overall enthalpies of the steps.

The overall reaction with Cl₂ is faster than Br₂

If excess halogen, e.g., Cl₂, more chlorination

i.e. 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

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 [CH₃CH₂CH₃][Cl· ] x fⁿ 6 H’s

Rate of reaction via 2° CH = k₂ [CH₃CH₂CH₃][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

CnH_2n+2 ® (3n +1)/2 O₂® nCO₂ + (n+1) H₂O + heat

H₄C + 2 O₂ ® CO₂ + 2 H₂O + D

e.g., cigarette lighter C₄H₁₀ + 6.5 O₂® 4CO₂ + 5H₂O
 

Alkane	DH combustion (kJ/mol)
CH4	213
H3CCH3	373
C4H10	687
(C4H8)	656 Ring strain
C5H12	845
(C5H10)	793

Alkyl Halides ? Haloalkanes

Chemistry controlled by bond polarity

^{d d -}C-Hdd⁺d⁺C-Xd^- 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

Reactions

1 Haloalkane Preparations; From alkanes

Seen above

3 From alcohols

2-propanol a substitution reaction

4 Addition to alkene

Reactions

5 Nucleophilic Substitution

Many e.g.’s of substitution reactions. (For OH^- need dilute solutions, see below)

6 Elimination of HX

7 Reactions with Group 1 or 2 metals Li / Mg

= Organometallic

S_N2 Reactions

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 S_N2 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.

Mechanism

The S_N1 Reaction

For 3° alkyl halides need very polar solvents and non basic nucleophiles to observe nucleophilic substitution. However, the kinetics are different.

(CH₃)₃CBr + N₃^_ ® (CH₃)₃C-N₃

a first order reaction; implies the overall reaction is not an elementary step (that the rate detemining step doesn’t need a nucleophile).

Why doesn’t the S_N2 happen with 3° alkyl halides? Two reasons – i) Steric reasons (space occupied). The alkyl groups block backside attack. ii) the 3° cation is sufficiently stable that another reaction pathway exists.

Order of stability of carbocations:

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 useful chemical test for 1° 2° and 3° alkyl halides

Some nucleophilic substitutions

Summary of 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

We saw above that OH- + an alkyl halide gives an alcohol. 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.

In this reaction, the HO^- is acting as base, not a nucleophile.

Rate Law; Rate = k[RX][OH-]; called E2 (elimination bimolecular)

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

7 Organometallics

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

H₃C^d+-Id- + Mg ® H₃C^d--Mg^d+-I methylmagnesium iodide

These are carbanions (very strong bases and nucleophiles). This is one of the few reactions we learn from which C-C bonds can be formed.

8 Reaction with water

9 Reaction with ketones or aldehydes

10 Reaction with CO₂

Alkenes

Preparation

6 From Haloalkanes

1-bromo-3-methylbutane 3-methyl-1-butene

11 From alcohols

an alcohol + an dehydrating acid (H₂SO₄, H₃PO₄)

cyclohexanol catalysts cyclohexane

b.p. 156 ° C b.p. 82 ° C

Alkene Reactivity

Dominated by p bonds

p bond \ energy (~ 267 kJ/mol strength of the double bond) more reactive than a s -bond ( 310 kJ/mol); It is a Lewis base - attack by E+ on p-electrons, i.e. In plane of screen above or below p -bond

These are extremely reactive almost as reactive as the metal

Fats: in the body: triglycerides

 

						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; b -carotene ® retinal (vision)

b -selinene, celery oil

myrcene, bay leaf oil

a -pinene, cedar leaf oil

Alkenes may exist in two different geometric isomers (see above) (Z- and E-)

Z-2-butene                      E-2-butene

Alkene Reactions

Electrophilic additions

Remember that the order of cation stability is:

3° ((CH₃)₃C⁺) > 2° ((CH₃)₂C⁺H) > 1° (H₃CC⁺H₂) > H₃C⁺

4 Acids and 12 alcohols

additions to alkenes usually proceed via the most stable cation.

get both cis and trans addition

13 Bromination

Can also use ICl (I+ Cl-)

Only get trans addition

14 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 a reduction

cis-addition of H₂, the reaction happens at the solid surface of Pt

Process used commercially to "hydrogenate" fats.

Oleic acid

Stearic acid is a saturated fat;

Not so good for you. Better for your health are polyunsaturated fats.

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.

Alcohols Preparation

5 S_N2 of alkyl halides with hydroxide

12 Hydration of alkenes

9 addition organometallic to aldehyde or ketone

electrophilic Nucleophilic

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

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

Note: CAN’T DO THE FOLLOWING REACTION.

Ethers are less polar than the alcohols (No OH’s for H-bonding)

They are made in a similar fashion to alcohols

5 Preparation of ethers

15 Making Alkoxides (Reducing Metals)

Alkoxides are strong bases and also good Lewis bases or nucleophiles

Recall: 2HO-H + 2K ® 2HO-K + H₂­

Reactions of Alcohols

3 Preparation of alkyl halides

Alcohols & halogen acids e.g. HCl, HBr, HI

Note that the reaction doesn’t work under basic conditions

The mechanism

Step 1 

Step 2 

11 Alcohol & Dehydrating Acid

e.g. 80% H₂SO₄, or H₃PO₄ is required

cyclopentanol Elimination

16 Alcohols + Oxidizing Agents

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

eg. C_rO₃ or K₂Cr₂O₇/H₂SO₄

Chromium IV or Na₃Cr₂O₇/H₂SO₄ º H₂CrO₄ or KMnO₄ (Mn^VII)

Chromic acid

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

1^o Alcohol C - CH₂OH

(a) with PCC

removes this H Ethanol (acetaldehyde)

(b) with K₂Cr₂O₇/H+ or KMnO₄

2^o Alcohol + Any oxidant

2-propanol 2-propanone

3^o Alcohol

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

Aldehydes & Ketones

Preparation

16 Oxidation of 1^o Alcohol

2-methylpropanol 2-methylpropanal

(2) Oxidation of 2^o Alcohol

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.

Examples

9 Organometallic + Ketone

aldehyde ® secondary alcohol

ketone ® alcohol

16 Reduction with NaBH₄

17 Reduction with H₂ and a P+ catalyst

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

18 Reaction with N-Compounds

e.g., hydrazine H₂NNH₂; 2,4-dinitrophenylhydrazine ; hydroxylamine H₂NOH

16 Oxidation - Aldehydes

Occurs very easily, while can use strong oxidants such as K₂Cr₂O₇/H+ or KMnO₄

Can also use weak oxidizing agents eg. Ag

Another more relevant oxidant – how to make a mirror

NOTE: Ketones + oxidants No reaction at normal temps

Carboxylic Acids

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

Ethanoic acid

The Structure Type

Is very common

R = any alkyl or aryl (aromatic)

e.g. 

Amides Esters Acid chlorides acetic anhydride

Preparation of Carboxylic Acids

10 Organometallic + CO₂

(2-Methylpropyl)lithuim

16 Aldehyde + any oxidant

2-phenylethanal 2-phenylethanoic acid

(Phenyl acetic acid)

1^o Alcohol + Any oxidant

ethanol ethanoic acid (acetic acid)

20 Reaction of -COOH with an Alcohol ® Esters

Need a strong acid catalyst

ethanoic acid ethyl ethanoate (ethyl acetate)

Reaction is a Nuc. Add followed by an elimination

(2) Acidity pKa» 3- 5

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.

Compare Alcohols

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 ca. 50