10. Organic Chemistry

Page 1

Fundamentals of Organic Chemistry

Naming Organic Compounds

1. Prefix: longest continuous carbon chain

1: meth-

2: eth-

3: prop-

4: but-

5: pent-

6: hex-

7: hept-

8: oct-

2. Identify type of bonds

Single: alkane

Double: alkene

Triple: alkyne

3. Suffix: Identify present functional groups

none: -e

-OH (hydroxyl): -ol

-NH2 (amine): amino-

-X (halogen): chloro-, bromo- or iodo-

(aldehyde): -al

(ketone): -one

(carboxyl): -oic acid

(ester): -oate

When there are multiple groups present,

arrange them in alphabetical order prior to the

root name.

4. Identify the side chains

Methyl group (1 carbon)

Ethyl group (2 carbons)

...

5. Use numbers and prefixes to indicate the number and location

of double/triple bonds or functional groups

→ Number is indicated by a prefix:

(mono-), di-, tri-, tetra-, penta-, hexa-

→Location is indicated with a number, and multiple numbers are

separated by a comma

eg. 2,2-dimethyl-propane

!! Always use lowest number possible (eg. 2 instead of 4 in

2-methyl-pentane)

Homologous series

Alkanes

→ general formula: C(n)H(2n+2)

Alkyl radicals R- are alkanes with one of the hydrogens removed

→ have a -1 charge

Homologous series have the same general formula with the

neighbouring members of the series differing by CH2

Primary, Secondary and Tertiary

Primary

→ Functional group is bonded to a carbon that is bonded to

only one other carbon (R group)

Secondary

→ Functional group is bonded to a carbon that is bonded to

two other carbons (R groups)

Tertiary

→ Functional group is bonded to a carbon that is bonded to

three other carbons (R groups)

Class of organic compound

Different compounds that contain the same functional group are

divided into classes.

→ Alcohols

→ Esters

→ Aldehydes

→ Ketones

etc.

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Alkanes

Properties of Alkanes

General formula

C(n)H(2n+2)

Low reactivity

→ Strong and stable C-C and H-C bonds, low polarity

→ Exceptions; undergo combustion and substitution reactions with

halogens under UV light

Combust fully to give carbon dioxide and water

→ Reaction is very exothermic; make good fuels

Even though chain alkanes are more common, they occur in ring structures

called cycloalkanes

Alkanes are fully saturated with hydrogens, contrary to alkenes and alkynes

!Boiling point increases as carbon chain length increases

Free Radical Substitution

Alkanes react with chlorine or other halogens in the presence

of UV light to form hydrogen chloride and a substituted alkane

→ Bromine is only halogen that does not need UV light to initiate reaction

Reaction mechanism; photochemical homolytic fission

→ heterolytic fission: both shared e go to one of the atoms

→ homolytic fission: each bonded atom retains one e; creation

of 2 free radicals (under influence of UV)

→ Mechanism steps:

1. Initiation; homolytic fission of halogen

2. Propagation; free radical halogen reacts with alkane

(eg. methane) to produce HX and a free radical methyl group

3. Termination; two free radicals react together (eg. radical methyl

and radical chlorine)

Halogenalkanes

→ more reactive than alkanes; can undergo nucleophilic substitution reactions

Page 3

Alkenes and Alkynes

Properties of Alkenes

Second homologous series of hydrocarbons

General formula: CnH2n

Contain at least one C=C bond

→ (sigma + pi bond)

→ sp2 hybridization

More reactive than alkanes

→ Undergo addition reactions

→ Unsaturated hydrocarbons

Can be distinguished from alkanes using bromine water,

or a halogen gas, which will decolourize in the presence of alkenes

Physical properties

→ non-polar or weak polar

→ no hydrogen bonding

→ Relatively low mp/bp ~alkanes

(Double bonds disrupt London forces)

→ Water insoluble

Properties of Alkynes

Third homologous series of hydrocarbons

General formula: CnH2n-2

Contain at least one triple carbon to carbon bond

→ sigma + 2 pi bonds

→ sp2 hybridization

Unsaturated hydrocarbons

→ reactive like alkenes (can also perform addition

reactions

Physical properties

→ Water insoluble

→ Non-polar or weak polar

→ Aroudnd the same boiling point of alkenes with

the same carbon chain lenght

Page 4

Alcohols

Alcohols are organic compounds that contain a hydroxyl group (-OH)

→ Compound names have the ending -ol

→ OH group is often isolated in molecular formula

Combustion

→ Combust completely to give carbon dioxide and water

→ Give out little pollution and doesn't contain sulphur

→ Obtainable from renewable sources

Oxidizing agents

→ Acidified potassium dichromate, K2Cr2O7

→ Potassium permanganate, K2Mn2O7

Primary Alcohols Oxidation

In primary alcohols, hydroxyl group is often found at the end of the chain

→ methanol (wood alcohol), ethanol (alcohol in wine)

→ oxidation products have lower boiling point than reactants; no hydrogen

bonding

1: Aldehyde

→ eg. methanal (formaldehyde)

→ Have a group with carbon with double bond to O and one bond to a H

→ To obtain aldehydes, it can be distilled from the reaction mixture as

soon as it is formed.

→ Polar molecules - dipole-dipole interaction

2: Carboxylic acid

→ eg. methanoic acid (formic acid), ethanoic acid (vinegar)

→ Have a C with double bond to O and OH (carboxyl group)

Secondary Alcohols Oxidation

1st oxidation: Ketone

→ eg. propanone

→ have a C double bonded to an O in the middle of the chain

→ Polar molecules - dipole-dipole interactions

! Ketones cannot undergo further oxidation

Tertiary Alcohols Oxidation

Any further oxidation unlikely with oxidizing agent, except for combustion

Hydroxyl substitution reactions

Alcohols can be made using a halogenalkane and -OH group

→ Halogen will be substituted by the hydroxyl group

eg. BrC2H7 + NaOH → C2H7OH + NaBr

Esters

An alcohol and an organic acid will react to an ester in a condensation reaction

→ Performed in water bath + concentrated sulfuric acid catalyst

→ Esters are compounds that often give a certain smell or taste

→ Polar molecules

Page 5

Structural isomers and Stereoisomerism

Structural isomers

→ Compounds with the same molecular formula but a

different structural formula

→ Similar chemical properties but possibly different

physical properties.

! The longer the carbon chain; the more structural isomers possible

Stereoisomerism

Cis-trans isomerism

→ Specific case of E/Z isomerism

→ Occur when rotation around bond is restricted or prevented

(eg. double/triple C-C bonds, pi-bond) when R1 and R2 are found

on either side of the double bond

→ Cis: both R groups on same side of double bond

(up-up / down-down)

→ Trans: R groups on opposite sides of double bond

(up-down / down-up)

→ Can also occur in cycloalkanes

E/Z isomerism

→ Occurs in every case where where free rotation around a C=C bond is restricted, could have 4 different R groups

→ Cahn-Ingold-Prelog (CIP) rules for determining the priority of the

atoms/groups attached to the two carbons

→ Z: both highest priority groups on same side (like cis, zusammen)

E: higher priorities lie on opposite sides (like trans, entgegen)

Physical and chemical properties of geometric stereoisomers

→ Same chemical but different physical properties, eg. boiling point

Optical Isomerism

→ All compounds that contain at leas one asymmetric or chiral carbon

within the molecule; contains 4 different atoms or groups.

→ This carbon is known as the stereocentre

→ Multiple stereocentres; multiple stereoisomers possible

Enantiomers: mirror images

Diastereomers: not mirror images

Enantiomers can rotate the plane of plane-polarized light

→ One turns it to the left, the other to the right

→ If a substance containes only the same enantiomers, the

substance will be optically active

- can be detected by a polarimeter

Page 6

Polymerization

Polymer

→ a long chain molecule produced by repeated chemical linkage (i.e. polymerization)

of many small molecules (monomers)

There are two types of polymers;

1. Addition polymers

→ formed by addition reaction of alkenes

2. Condensation polymers

→ formed by condensation reaction

Addition Polymers

Addition reaction

→ weak pi bond of alkene (C=C) monomer 'opens' allowing many (n)

C-C units to form long chains

→ Occur with catalyst + high pressure and temperature

Peroxides (free radical mechanism) or Zeigler catalyst (carbocation mechanism)

→ Most plastics are addition polymers, but little to no biological systems know how

to break down these polymers

Addition polymer properties

→ Alkane-like

→ Nonreactive (except in combustion)

→ Non-biodegradable: cause pollution and environmental harm

Condensation Polymers

Condensation reaction

→ Elimination of simple molecules (H2O, HCl)

→ Conditions: catalyst + water bath

dicarboxylic acid + diol → polyester + H2O

diacylchloride + diamine → polyamine + HCl

Condensation polymers

→ 2 functional groups needed per monomer; one at

each end of a chain structure

Polyesters

→ Form very long chains; fibres used for clothing

→ n Diol + n dicarboxylic acid → [polyester]n + (n-1) H2O

Polyamides

→ condensation reaction: n dicarboxylic acid +

n diamine → [polyamide]n + (n-1) H2O

→ form long-chain fibres used for clothing

Note: ester + amide bonds are hydrolysable

→ more biodegradable than addition polymers

ie. polyester + (n-1) H2O → n diacid + n diol

→ Will degrade in nature but release organic

residues into environment

Condensation polymer examples

Poly(lactic acid), PLA

→ Made from a single monomer, lactic acid

→ Renewable resource; can be obtained through

fermentation of starch or sugar, biodegradable

→ Can undergo repeated condensation reaction

with itself

KEVLAR

→ Used for bulletproof vests

→ a polyamide (aramid) with low density and very

high tensile strength; used for body armour, sports

equipment, brake linings, ropes

Page 7

Nucleophilic Substitution Reactions

Key pattern → one bond breaks at C, other bond forms at C

In nucleophilic substitution reactions, there are 2 types:

Sn1 → waits for the other atom to leave

Sn2 → attacks and kicks off other atom

Nucleophile

→ Groups with electron-rich atoms able to donate a pair of

electrons (lewis bases)

→ Ion/molecule which is strongly attracted to a region of

positive charge. They are fully negative or have a strong

negative charge somewhere.

→ Examples:

-OH hydroxides

-CN cyanides

H2O water (weak)

NH3 ammonia (weak)

HBr halogenacids

Sn1 Reactions

In Sn1, the nucleophile 'waits' for the bonded halogen group to

leave before reacting with the remaining carbocation

→ 2 step reaction:

1. Leaving of halogen - slow

2. nucleophile bonding with carbocation - fast

→ Can happen from both sides; 2 optical isomers formed

Product is not optically active

Sn2 Reactions

In Sn2 reactions, the nucleophile 'attacks' from the opposite side of the

molecule and forces halogen group to leave

→ Only 1 optical isomer is formed; product is optically active

enantiomer is inverse of reactant

→ X leaves at the same time as the nucleophile bonds; 1 step reaction

with transition state

Factors affecting Sn1 or Sn2

The 'Big Barrier'

→ key factor that prevents the reaction from happening

Sn1: carbocation stability increases as there are more

bonds on the central carbon

3 > 2 >> 1

Sn2: steric hindrance

Nucleophile has to attack from opposite side

1 > 2 >> 3

Solvent characteristics

→ Sovents can be nonpolar, polar protic or polar aprotic

polar protic solvents: contain an -OH group, which makes

them effective in stabilizing the carbocation

> Sn1 is favoured, Sn2 disfavoured

Nucleophile type

→ Weak nucleophile: Sn1

→ Strong nucleophile: Sn2

Rate law

→ Sn1 is a 2 step reaction where the first step is the slow step;

Rate law depends on substrate only

→ Sn2 is a single step reaction

Rate law depends on both substrate and nucleophile

Page 8

Elimination reactions

Key pattern → substrate with a base forms an alkene + conjugate

acid + leaving group

Factors affecting elimination or substitution

Temperature

→ At lower temperatures, nucleophilic substitution occurs

→ At higher temperatures, elimination will occur

→ Often a combination between these two, but the higher

T, the more elimination products

Explanation:

Substitution forms 2 products while elimination forms 3

products → greater increase in entropy

We have that

G = H - TS

At higher temperatures, Gibbs Free energy is more negative

for elimination reactions than substitution

Page 9

Electrophilic Addition and Substitution Reactions

Electrophilic Addition

Key pattern → a pi bond is broken

Electrophile

→ attracted to a region of high electron density

Addition to asymmetrical alkenes

In theory, two isomers could be produced in an

addition to an asymmetrical alkene. However,

there is often one that is much more favoured.

Markovnikov's observation

→ There is a preference for secondary halogenated

alkanes

→ The hydrogen goes to the C with the most hydrogens

bonded to it

Markovnikov's rule

'In an addition reaction of a protic acid HX to an alkene

or an alkyne, the hydrogen atom becomes bonded to

the carbon that had the greatest number of hydrogens

Explanation:

Stability of carbocation when pi-bond breaks:

3 > 2 >> 1

Akyll groups tend to push electrons towards the carbon

atom they are bonded to: stabilizes charge on

carbocation

Electrophilic Substitution

Benzene is the simpelest aromatic hydrocarbon compound (arene)

and has a delocalized structure of pi-bonds around its ring

> each C-C bond has order of 1.5

> Susceptible to attack by electrophiles

> Does not readily undergo addition but does substitution

Nitration of Benzene

Benzene reacts with a mixture of concentrated nitric acid and

concentrated sulfuric acid when warmed at 50oC to give nitrobenzene

and water