1Chemistry 1102Charlie BondMCS Rm 4.16/4.27Charles.Bond@uwa.edu.au

What is Organic Chemistry?Organic Reactions I IIAlkanes (Ch 3)Conformational Analysis (Ch 3)Stereochemistry I II III (Ch 6)Alkyl Halides I II (Ch 7)Alcohols and Ether I II (Ch 8)

2To write mechanisms for organic chemistry, we show the flow of electronsfrom electron-rich species to electron-poor species with curly arrows.

Placement of the curly arrow is important!It originates at the electron pair and terminatesat the electron-poor atom

Example: - bond makingWrite reaction mechanism between hydroxide ion and proton:

Reaction Mechanisms

a curly arrow represents the movement of two electrons

O H O H

3Example:Example: bond breakingbond breaking

Reaction MechanismsReaction Mechanisms

C Cl C Cl

carbone lost oneelectron and becomes positively charged

chlorine had gained oneelectron and becamesnegatively charged

curly arrow is directed from single bond to electronegativeatom

Lone pairs:Lone pairs: nonbonding electrons can also be used in bond-making process.nonbonding electrons can also be used in bond-making process.

Write the reaction between water and a proton:Write the reaction between water and a proton:

H

an oxoniumcation

HO

HH O H

H

C loses one C loses one electron and electron and becomes positively becomes positively chargedcharged

Cl gains an electron Cl gains an electron and becomes and becomes negatively chargednegatively charged

4Reaction Mechanisms

CARBON

- has valency of four

- carbon can also bond to just three other atoms by donating a pair of electrons to one of the atoms originally bonded (breaking the bond)

C

tetravalent carbon

C X C

CARBOCATIONcarbone lost one

electron and becomes positively charged

curly arrow is directed from single bond to electronegativeatom

X

HC H C

CARBOANION

CARBOCATION

Carbon loses one electron and becomes +ve

CARBANION

5NITROGEN- has valency of three- has three bonding electrons and a lone pair

- however it can also bond to four atoms by donating its lone pair, in which case it will than carry a positive charge.

Write reaction between ammonia and proton:

H NH

H H H NH

HH

trivalent nitrogen an ammoniumcation

What shape is ammonia?

What shape is an ammonium ion?

6OXYGEN- has two bonding electrons and two lone pairs- it can bond to two other atoms and it is usually divalent

- it can also bond to one atom in a negatively charged form, or to three atoms in positively charged form.

O

divalent oxygenH

an oxide anion

HO O

H

O OH

an oxonium cation

7BOND POLARITY

- because different atomic nuclei electronegativity. This leads to charge imbalance, with one of the atoms taking more than its share of electrons.

- an atom that is more electronegative than carbon will polarize the bond, this is indicated by putting partial charge + and - above the atoms. It can also be represented by putting an arrowhead on the bond in the direction of electron excess.

H3C Br+

bromine is more electronegative than carbon

H3C Br

+ C O

+ C N

polarity in C-O and C-N bonds

+ C O

8Nucleophiles, Electrophiles and Leaving GroupsNucleophiles, Electrophiles and Leaving Groups

Reagents are classified as nucleophiles or electrophiles. Nucleophiles are electron rich, nucleus-seeking reagents and typically have a negative charge (anions) or a lone pair.

charged nucleophiles (anions)

R O Br C C N

uncharged nucleophiles (lone pair)

H2O R NH2 R SH

9Electrophiles are electron-deficient, electron-seeking reagents, and typically have positive charge (cations) or are polarisable molecules that can develop an electron-deficient centre.

+ C O

uncharged electrophiles (polarisation)

+ C Br

Many reactions will involve both nucleophiles and electrophiles. The electron-rich species is always regarded as the attacking agent.

Nu E Nu E

H

charged electrophiles (cations)

C O N O

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Leaving group is the terminology used for ions or neutral molecules that are displaced from a reactant as part of a mechanistic sequence. Frequently this displacement is the consequence of a Nu- attacking an E+ (E+ carries a suitable leaving group).

C L+ Nu

nucleophile

electrophile

Nu C L

leaving group

Good leaving groups are those that form stable ions or molecules after they leave molecule.

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While you are learning: Draw structures in full Identify what has changed from reactant(s) to product(s) Identify Nu: and E+

Show all lone pair electrons on atoms involved in reaction Connect Nu: and E+ with curly arrow (colour is a good idea) Do any atoms violate the octet rule? If so some bonds

must be broken in the same step. Is the mechanism balanced (charge and atom)? Curly arrows always move in the same direction never

meet head on.

Tips for Writing Mechanisms

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Problem:

Order the following reagents into the nucleophiles and electrophiles:

H3C O Br

H2O

NH2

H3C SH

H

CH3CH3CCH3

O N O

H3C Br1 2 3 4

5

6

7 8 9

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Resonance For many molecules and ions, no single

Lewis structure provides a truly accurate representation

andOCO

CH3 OCO

CH3

14

Resonance Examples

NO

ONO

O

Ethanoateion(equivalentcontributingstructures)

CO

OCH3 C

O

OCH3

Nitriteion(equivalentcontributingstructures)

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Q. Write the mechanism for the reaction of aqueous HCl with water

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Reaction MechanismsQ. Write the mechanism for the reaction of:

1. hydroxide with CH3Cl

2. fluoride with Me3O+

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Resonance Draw the resonance structures of the

carbonate ion CO32-

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