Organic ChemistryMarch/April 2015

+ [O], heat

reflux

HalidesPrimary Carbons

Secondary Carbons

Tertiary Carbons

IMPORTANCE?? REACTIONS!!

Substitution Reactions of Alkanes - Halogenation

CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)UV LIGHT

Reaction Mechanism - sequence of steps in a chemical reaction

Initiation - creating the free radical

Propagation - both use and produce free radicals

Termination - removing free radicals from the mixture

SHOW THE MECHANISM

Alkenesdouble bonded carbons - one sigma bond and one pi bond

Addition Reaction - attacks the weaker pi bond, therefore can be broken, leaving the pi bond

Hydrogenation

CH2CHCH3 + H2 CH3CH2CH3

Ni catalyst @ 150°C

With Halogens...

CH2CHCH3 + Cl2 CH2ClCHClCH3

With hydrogen halides

CH2CHCH3 + HCl CH2ClCHClCH3

With water

mechanism in GoodNotes

Alkanes vs Alkenesalkenes undergo addition where alkanes will not

different flame colors

burning of alkenes is dirtier - higher C:H ratio

Aromatic Compounds - Benzene Ring

Highly unsaturated - very smoky flame

very high in residual C

BenzeneC6H6

Aromatic functional group -

C6H5 (IUPAC name - phenyl)

Polymerization

-alkenes readily undergoing addition to create long chains

mechanism for chloroethane...

Challenge

Draw the repeating unit of polystyrene if the formula of the monomer is C6H5CHCH2

Alcohols-OH functional group - polar - increases volatility and solubility of alkanes with similar molecular mass

most common alcohol - C2H5OH - ethanol - readily soluble in water

don’t forget your lone pairs

Oxidation of alcohols (different than combustion)

when looking to oxidize an alcohol, use an oxidizing agent (acidified potassium dichromate)

Primary Alcohol Oxidation - two step

to an aldehyde first, then to carboxylic acid

+ [O], heat + [O], heat

reflux

Secondary Alcohols-oxidized to ketones by a similar process

+ [O], heat

reflux

CH3CHOHCH3

propan-2-ol(CH3)2CO

propanone

Tertiary Alcoholsnot readily oxidized undercomparable conditions

would involve breaking the carbon skeleton

LOTS OF ENERGY!!!

+ [O], heat

refluxNO RXN!!

HalogenoalkanesSaturated, polar molecules

the carbon in this molecule is considered “electron poor”

nucleophile - reactants that are electron rich and hence attracted to regions of electron deficiency

leads to a reaction where the substitution of the halogen occurs

called Nucleophilic Substitution

shorthand - Sn

Primary Halogenoalkanes - SN2 Mechanism

Overall reaction occurs with NaOH:

CH3Cl + OH- CH3OH + Cl-

Since hydrogen atoms are small, there are no “blockers” for the carbon to defend itself

Tertiary Halogenoalkane: SN1 Mechanism

The presence of 3 alkyl groups...what is happening?

...this is called steric hindrance - bulky carbon atoms make it difficult for an incoming group to attack the carbon atom

Carbon cation Stability

Secondary Halogenoalkanes...?

What type of substitution do you think they undergo?

SN1 SN2

Nucleophilic Substitution ReactionsOrganic Reactions are very sloooooooooooowww

Rate of reaction is important and studied heavily

Experiments show that:

SN1 is faster than SN2 reaction...why??

Which is the fastest rate of reaction...?

(primary/secondary/tertiary?)

TERTIARY > SECONDARY > PRIMARY

Nucleophilic Substitution Reactions

Effect of Halogen on Nuc. Sub. Rxns

Two Opposing Factors to consider

1. Polarity of the C-H bond

2. Strength of the C-H bond

Nucleophilic Substitution ReactionsPolarity - electronegativity decreases down a group (from F to Cl to Br to I)

Carbon is less and less electron deficient - less vulnerable to nucleophilic attack

Strength - as bond strength decreases down the group, the breakage of this bond is easier (iodoalkane is the most reactive, fluorine is the least)

Nucleophilic Substitution ReactionsBased on the reaction data:

Strength is more important

Relative rate of reaction - -

iodoalkanes > bromoalkanes > chloroalkanes > fluoroalkanes

Nucleophilic Substitution ReactionsThese reactions are vital to many organic reactions

Allow for different synthetic reagents to be used

Conversions from halides to:

Alcohol

Amine

Nitrile

Good notes time!!

Conversion into AlcoholBromoethane and NaOH --> ethanol and NaBr

Class of alcohol produced will match class of halide used in reaction

This will work with water, but is better with a base...

WHY?!?!

Elimination ReactionsBasic reaction:

Halogenoalkane minus a hydrogen and a halogen

Elimination Reactions

E1 Mechanism - less common

Good notes time!!

Elimination ReactionsMore common Elimination reaction --> E2

Halogenoalkanes undergo both SN and E reactions

Nucleophilic substitution is more common

-NaOH in a warm, aqueous solution where OH- acts as a nucleophile

Elimination reactions are favored when:

NaOH is dissolved in a hot alcohol where it behaves as a base (grabbing H+ to form water)

Condensation reactionsWhen two molecules react to form a product with the loss of a small molecule such as water, HCl, or ammonia

Also called addition-elimination reactions

Good notes time!!

Condensation PolymersInclude:

Natural molecules such as silk and DNA

Synthetic molecules such as Nylon, Kevlar and PET - polyethylene terephthalate

Different from addition polymerization where no small molecule is released

Esterification

Carboxylic acids and alcohols condense to form esters

Good notes time!!

Brain Teaser

If you had a condensation polymer of an ester, what would it be called???

POLYESTERS!Good notes

time!!

Forming Amides

Carboxylic acids and amines

Good notes time!!

Reaction PathwaysDiagram on page 406...

Need to be able to go from one compound to the next...

+

+ [O], heat

StereoisomerismBuild 2-butene....GO!

Are all of our structures the same?

2 types of stereoisomerism

Geometric - cis and trans isomers

Optical isomers

Geometric Isomers-occur with some constraint

Now, you try it!

Draw and name the geometric isomers of butenedioic acid. (Be careful...there are two tricks!)

Good notes time!!

Physical PropertiesDepend on:

Polarity

Shape or symmetry

Name these:

What intermolecular forces will these undergo?

Which will have a higher boiling point?

Optical IsomersA carbon atom attached to four different atoms or groups is known as asymmetric or chiral

Optical IsomersCan be arranged in two different 3-D configurations that are mirror images of each other

Known as optical isomerism

The term refers to the way the isomers react with plane polarized light

They are chiral molecules with NO plane of symmetry

Plane of symmetry

Try it with your hands...

Enantiomers

The two non-superimposable forms of optical isomers

A mixture containing equal amounts of the two is known as a RACEMIC mixture

A racemic mixture is optically inactive...

How do I know it's an optical isomer?

Find the C atom with four different groups bonded to it. It is useful to mark the CHIRAL carbon with an asterisk.

Try it!! Draw butan-2-ol. Find the chiral carbon. Mark it with an asterisk. Good notes

time!!

Good notes time!!

Draw 2-chlorobutane. Find the chiral carbon.

Properties of Optical Isomers

Have identical physical and chemical properties

Except:

Optical activity

Reactivity with other chiral molecules