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Organic Chemistry

By Dr.Ahmed AbdoulAmier Hussain Al-Amiery

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Electronic configuration of Carbon

C 1s2 2s2 2p2 • Covalent bonds: sharing of electrons between atoms • Carbon: can accept 4 electrons from other atoms • i.e. Carbon is tetravalent (valency = 4)

Ethane: a gas (b.p. ~ -100oC Empircal formula (elemental combustion analysis): CH3 Measure molecular weight (e.g. by mass spectrometry): 30.070 g mol-1, i.e (CH3)n n = 2 Implies molecular formula = C2H6 Molecular formula: gives the identity and number of different atoms comprising a molecule Ethane: molecular formula = C2H6 Valency: Carbon 4 Hydrogen 1 Combining this information, can propose

structural formula for ethane

• Each line represents a single covalent bond • i.e. one shared pair of electrons

• Structural formulae present information on atom-to-atom connectivity

C CH

HH

H

HH

C C

H

H

HH

H

H

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• However, is an inadequate represention of some aspects of

the molecule

• Suggests molecule is planar • Suggests different types of hydrogen

Experimental evidence shows: • Ethane molecules not planar • All the hydrogens are equivalent • 3 Dimensional shape of the molecule has tetrahedral

carbons • Angle formed by any two bonds to any atom = ~ 109.5o

Angle between any two bonds at a Carbon atom = 109.5o

109.5°

C C

H

H

H

H

H

H

109.5o

C C

H

H

H

H

H

H109.5o

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Electronic configuration of Carbon C 1s2 2s2 2p2 Hydrogen H 1s1

• However, know that the geometry of the Carbons in ethane is tetrahedral

• Cannot array py and pz orbitals to give tetrahedral geometry

• Need a modified set of atomic orbitals - hybridisation

C CH

HH

HH

HEthane

Orbitals available for covalent bonding?

H 1s(1 e ) C 2py

(vacant)

C 2pz

(vacant)

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Bonding in ethane Atomic orbitals available: 2 Carbons, both contributing 4 sp3

hybridised orbitals 6 Hydrogens, each contributing an s orbital

Total atomic orbitals = 14 Combine to give 14 molecular orbitals 7 Bonding molecular orbitals; 7 anti-bonding molecular orbitals Electrons available to occupy molecular orbitals One for each sp3 orbital on Carbon; one for each s orbital on Hydrogen Just enough to fully occupy the bonding molecular orbitals Anti-bonding molecular orbitals not occupied Ethane: molecular orbital diagram

δ molecular orbitals: symmetrical about the bond axis Four sp3 hybridised orbitals can be arrayed to give tetrahedral geometry,

σCH

σ*CH

σCC

σ*CC

Energy C C

H

H

H

H

H

H

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sp3 hybridised orbitals from two Carbon atoms can overlap to form a Carbon-Carbon s bond

An sp3 orbital extends mainly in one direction from the nucleus and forms bonds with other atoms in that direction.

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This represents a particular orientation of the C-H bonds on adjacent Carbons

Newman projection

Staggered conformation: Minimum energy conformation (least crowded possible conformation) Eclipsed conformation: Maximum energy conformation (most crowded possible conformation)

• Eclipsed conformation experiences steric hindrance

H

H H

H

H

H

H

H H

H

H

H

H

HH

H

HH

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• Unfavourable interaction between groups which are close together in space

• These unfavourable interactions absent in the staggered conformation

• Hence, the staggered conformation is lower in energy • Energy difference between eclipsed and staggered

conformations of ethane = 12 kJ mol-1 • Each C-H eclipsing interaction contributes 4 kJ mol-1 of

torsional strain energy

Conformations: different orientations of molecules arising from rotations about C-C s bonds Consider one full rotation about the C-C bond in ethane Start at φ = 0° (eclipsed conformation)

Staggered conformation strain energy 0 kJ mol-1 Eclipsed conformation strain energy 12 kJ mol-1 Identical to that at φ = 0° Hence, in one full rotation about the C-C bond

• Pass through three equivalent eclipsed conformations (energy maxima)

H

HH

H

HH

4 kJ mol-1

4 kJ mol-14 kJ mol-1

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• Pass through three equivalent staggered conformations (energy minima)

• Pass through an infinite number of other conformations

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Some points concerning this series of alkanes 1. Series is generated by repeatedly adding ‘CH2’ to the revious member of the series. A series generated in this manner is known as an homologous series

2. Nomenclature (naming): Names all share a common suffix, i.e.’ …ane’ The suffix ‘…ane’ indicates that the compound is an alkane The prefix indicates the number of carbons in the compound

3. Representation and conformation :

C CH

HH

H

HCH

HCH

HH

Butane(full structural formula)

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• Structural formulae: give information on atom-to-atom connectivity

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Butane CH3-CH2-CH2-CH3

One full 360° rotation about the central C-C of butane: Pass through three staggered and three eclipsed conformations

H

HH

H

CH3

H HH

H

H

CH3

H

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Alkenes Degree of Unsaturation n Relates molecular formula to possible structures n Degree of unsaturation: number of multiple bonds or rings n Formula for saturated a acyclic compound is CnH2n+2 n Each ring or multiple bond replaces 2 H's

Example: C6H10 n Saturated is C6H14

n Therefore 4 H's are not present n This has two degrees of unsaturation

n Two double bonds? n or triple bond? n or two rings n or ring and double bond

Degree of Unsaturation With Other Elements n Organohalogens (X: F, Cl, Br, I)

n Halogen replaces hydrogen n C4H6Br2 and C4H8 have one degree of unsaturation n Oxygen atoms - if connected by single bonds

CH3 CCH3

CH3

CH2 CCH3

HCH3

2,2,4-Trimethylpentane

CH3 CH2 CCH3

CH2

CH2 CCH3

CH3

CH3 H 1

23456

2,4-dimethyl-4-ethylhexane

H3CC

CC

CCH3

H H

H H

H H

H H

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n These don't affect the total count of H's

Degree of Unsaturation and Variation n Compounds with the same degree of unsaturation can have

many things in common and still be very different

Summary - Degree of Unsaturation n Count pairs of H's below CnH2n+2 n Add number of halogens to number of H's (X equivalent to

H) n Don't count oxygens (oxygen links H) n Subtract N's - they have two connections

Naming of Alkenes n Find longest continuous carbon chain for root n Number carbons in chain so that double bond carbons have

lowest possible numbers n Rings have “cyclo” prefix

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Many Alkenes Are Known by Common Names: n Ethylene = ethene n Propylene = propene n Isobutylene = 2-methylpropene n Isoprene = 2-methyl-1,3-butadiene

Alkene Nomenclature

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Common names

Isomers and stability

Cis-Trans Isomerism in Alkenes n The presence of a carbon-carbon double can create two

possible structures n cis isomer - two similar groups on same side of the

double bond n trans isomer similar groups on opposite sides

n Each carbon must have two different groups for these isomers to occur

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Naming stereoisomers: The E-Z system (vs. cis- and trans-)

Electronic Structure of Alkenes n Carbon atoms in a double bond are sp2-hybridized

n Three equivalent orbitals at 120º separation in plane n Fourth orbital is atomic p orbital

n Combination of electrons in two sp2 orbitals of two atoms forms σ bond between them

n Additive interaction of p orbitals creates a π bonding orbital n Subtractive interaction creates a π anti-bonding

orbital n Occupied π orbital prevents rotation about σ-bond n Rotation prevented by π bond - high barrier, about 268

kJ/mole in ethylene

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Rotation of π Bond Is Prohibitive

n This prevents rotation about a carbon-carbon double bond

(unlike a carbon-carbon single bond). n Creates possible alternative structures

Alkene Stability: n Cis alkenes are less stable than trans alkenes n Compare heat given off on hydrogenation: ∆Ho n Less stable isomer is higher in energy

n And gives off more heat n tetrasubstituted > trisubstituted > disubstituted >

monosusbtituted n hyperconjugation stabilizes alkyl

Comparing Stabilities of Alkenes n Evaluate heat given off when C=C is converted to C-C n More stable alkene gives off less heat

n Trans butene generates 5 kJ less heat than cis-butene

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Preparation of Alkenes

Dehydration of Alkenes

Regioselectivity in alcohol dehydration: Zaitsev’s Rule

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Stereoselectivity in ROH dehydration

E1 and E2 Mechanisms of alcohol dehydration n Use what you know to predict the mechanism for this

reaction

n Resembles ROH + HX n Both promoted by acids n Reactivity rate 3º > 2 º > 1 º

Same thing:

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Rearrangements: methyl shift:

Rearrangements: hydride shift

Dehydrohalogenation of R-X

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Other bases to use: n NaOCH3 in MeOH n KOH in EtOH n For primary alcohols

q KOC(CH3)3 in t-BuOH or DMSO E1 mechanism: n More likely for 3º RX

q And for RI > RCl q And weak or no bases q Solvent acts as base (EtOH) q Often show rearrangements (E2 not as much) q Favors mixtures of products q So to increase yield of single product, avoid E1

E2 mechanism proofs:

1. Rate = k[RX][base], 1. favored for strong bases 2. Any RX

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Anti E2 : Stereoelectronic effects

Cis- isomer 500x faster rate than trans-

Reaction of Alkenes: Reaction with X2 Reaction with X2 in H2O

Cl

Cl

Br

Br

Cl2

Br2

OH

Br

Br2/H2O

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Reduction of Alkenes: Hydrogenation

Mechanism of Catalytic Hydrogenation

Electrophilic Addition of HX to Alkenes: n General reaction mechanism: electrophilic addition n Attack of electrophile (such as HBr) on π bond of alkene n Produces carbocation and bromide ion n Carbocation is an electrophile, reacting with nucleophilic

bromide ion Writing Organic Reactions n No established convention – shorthand n Can be formal kinetic expression n Not necessarily balanced n Reactants can be before or on arrow n Solvent, temperature, details, on arrow

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Example of Electrophilic Addition: n Addition of hydrogen bromide to 2-Methyl-propene n H-Br transfers proton to C=C n Forms carbocation intermediate

n More stable cation forms n Bromide adds to carbocation

Orientation of Electrophilic Addition: Markovnikov’s Rule: n In an unsymmetrical alkene, HX reagents can add in two

different ways, but one way may be preferred over the other

n If one orientation predominates, the reaction is regiospecific

n Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents)

This is Markovnikov’s rule Example of Markovnikov’s Rule n Addition of HCl to 2-methylpropene n Regiospecific – one product forms where two are possible n If both ends have similar substitution, then not

regiospecific

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Energy of Carbocations and Markovnikov’s Rule n More stable carbocation forms faster n Tertiary cations and associated transition states are more

stable than primary cations

Mechanistic Source of Regiospecificity in Addition Reactions: n If addition involves a carbocation intermediate

n and there are two possible ways to add n the route producing the more alkyl substituted

cationic center is lower in energy n alkyl groups stabilize carbocation

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Carbocation Structure and Stability: n Carbocations are planar and the tricoordinate carbon is

surrounded by only 6 electrons in sp2 orbitals n The fourth orbital on carbon is a vacant p-orbital n The stability of the carbocation (measured by energy

needed to form it from R-X) is increased by the presence of alkyl substituents

n Therefore stability of carbocations: 3º > 2º > 1º > +CH3 Transition State for Alkene Protonation: n Resembles carbocation intermediate n Close in energy and adjacent on pathway n Hammond Postulate says they should be similar in

structure

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Mechanism of Electrophilic Addition: Rearrangements of Carbocations: n Carbocations undergo structural rearrangements following

set patterns n 1,2-H and 1,2-alkyl shifts occur n Goes to give more stable carbocation n Can go through less stable ions as intermediates

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Oxidation of Alkenes: Epoxides: Epoxide Preparation

Cl

OOHO

O

H

mcpba

CH2Cl2

mcpba = peroxide

OH

BrO

Br2/H2O base

bromohydrin

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Radical Reactions – HBr

• If reaction is done with HBr/peroxides • Get the non-Markovnikov product

Radical Reactions: Polymer Formation:

• Polymer – a very large molecule made of repeating units of smaller molecules (monomers)

• Biological Polymers • Starch • Cellulose • Protein • Nucleic Acid

Polymers: • Alkene polymerization • Initiator used generally is a radical

Mechanism:

• Initiation • Propagation • Termination

Br

HBr/peroxides

n

repeatingunit

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Alkynes Reactions of Alkynes:

• Alkynes are rare in biological chemistry • Chemistry is similar to alkenes • Generally less reactive than alkenes • Reactions can be stopped at alkene stage using one

equivalent of the reagent Reactions with HX

• Regiochemistry is Markovnikov

Reactions with X2

• Initial addition gives trans intermediate • Product with excess reagent is tetra-halide

Reactions with H2

• Reduction using Pd or Pt does not stop • Alkene is more reactive than alkyne

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Reactions with H2

• Lindler’s catalyst is poisoned • Not as reactive • Stops at cis-alkene

Reduction using dissolving metals

• Anhydrous ammonia (NH3) is a liquid below -33 ºC • Alkali metals dissolve in liquid ammonia • Provide a solution of e- in NH3 • Alkynes are reduced to trans alkenes with sodium or

lithium in liquid ammonia Hydration of Alkynes

• Hydration (Hg+2) of terminal alkynes provides methyl ketones

• Hydration (BH3) of terminal alkynes provides aldehydes

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Alkyne Acidity: Acetylide Anion • Terminal alkynes are weak Brønsted acids • pKa is approximately 25 • alkenes and alkanes are much less acidic • Reaction of strong anhydrous bases with a terminal

acetylene produces an acetylide ion

Alkylation of Acetylide Anions

• Acetylide ions are nucleophiles • Acetylide ions are bases • React with a primary alkyl halides

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Aromatic Compounds Discovery of Benzene

• Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1.

• Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C6H6.

Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic Kekulé Structure

• Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested.

• Failed to explain existence of only one isomer of 1,2-dichlorobenzene.

Resonance Structure Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring.

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Unusual Reactions • Alkene + KMnO4 → diol (addition)

Benzene + KMnO4 → no reaction. • Alkene + Br2/CCl4 → dibromide (addition)

Benzene + Br2/CCl4 → no reaction. With FeCl3 catalyst, Br2 reacts with benzene to form bromobenzene + HBr (substitution!). Double bonds remain Unusual Stability Hydrogenation of just one double bond in benzene is endothermic

Annulenes

• All cyclic conjugated hydrocarbons were proposed to be aromatic.

• However, cyclobutadiene is so reactive that it dimerizes before it can be isolated.

• And cyclooctatetraene adds Br2 readily. Look at MO’s to explain aromaticity

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MO Rules for Benzene

• Six overlapping p orbitals must form six molecular orbitals.

• Three will be bonding, three antibonding. • Lowest energy MO will have all bonding interactions, no

nodes. As energy of MO increases, the number of nodes increases. MO’s for Benzene

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Energy Diagram for Benzene

• The six electrons fill three bonding pi orbitals. • All bonding orbitals are filled (“closed shell”), an

extremely stable arrangement.

Aromatic Requirements

• Structure must be cyclic with conjugated pi bonds.

• Each atom in the ring must have an unhybridized p orbital. • The p orbitals must overlap continuously around the ring.

(Usually planar structure.) Compound is more stable than its open-chain counterpart. Anti- and Nonaromatic

• Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but the energy of the compound is greater than its open-chain counterpart.

Nonaromatic compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar. Hückel’s Rule

• If the compound has a continuous ring of overlapping p orbitals and has 4N + 2 electrons, it is aromatic.

If the compound has a continuous ring of overlapping p orbitals and has 4N electrons, it is antiaromatic.

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[N]Annulenes • [4]Annulene is antiaromatic (4N e-’s) • [8]Annulene would be antiaromatic, but it’s not planar, so

it’s nonaromatic. • [10]Annulene is aromatic except for the isomers that are

not planar. Larger 4N annulenes are not antiaromatic because they are flexible enough to become nonplanar. Cyclopentadienyl Ions

• The cation has an empty p orbital, 4 electrons, so antiaromatic.

The anion has a nonbonding pair of electrons in a p orbital, 6 e-

’s, aromatic.

Common Names of Benzene Derivatives

OH OCH3NH2CH3

phenol toluene aniline anisole

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CH

CH2 C

O

CH3C

O

HC

O

OH

styrene acetophenone benzaldehyde benzoic acid