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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
•
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