lecture 1-2 suvarn_addition to c=o, mo aspects

8/11/2019 lecture 1-2 Suvarn_Addition to C=O, MO Aspects

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CH222CH222--Organic Chemistry 2Organic Chemistry 2

Wednesdays

&

Fridays11.05 am (sharp) to 12.30 pm

27

Lectures

1.5

h

each

(18

+

22

h)

Includes

problems

solving/

tutorials u zzes

1st quiz

wed

30

Jan

nd th

Mid sem exam 18 Feb

End

sem exam

19

Apr

Suvarn Kulkarni



•

• God created earth and heaven

• n

• The cabronyl group



Syllabus

• Addition to CO group, MO • Enols, enolates and reactions

aspects

• Organometallic reagents for

C-C bond formation

• Aldol reaction• Acylation

•

• Conjugate addition reactions

• Chemoselectivity: Oxidation

and reduction

• Stereoselective reactions

• Organo borane and silicon

reagents

• Retros nthetic anal sis



CH-222 Organic Chemistry II

Reactions and Mechanisms

• Nucleophilic Addition to C=O group: MO aspects,

Hydration and cyanohydrin reaction, thermodynamic Vs.Kinetic control, Organometallic reagents for C-C bond

,

compounds), ortholithiation, transmetallation, metal

halogen exchange, Stereochemistry of nucleophilica on o car ony group, on uga e a on reac ons.

group, carbonyl reactivity, hydrolysis of esters, amides

and nitriles, esterification, transesterification, conversion

o ac n o ac er va ves, ma ng e ones an

aldehydes from esters via weinreb amide and others.



• Nucleophilic substitut ion at C=O with the loss of carbonyl

oxygen: acetals, cyclic acetals, thioketal, imines, enamines,

re uc ve am na on, a erna ve me o s or am nes

formation, LAH reduction of amides, Strecker synthesis of

amino acids, Wittig reaction

• Enols, enolates and reactions: Alkylation, kinetic and

thermodynamic enolates, Aldol reaction, specific enol

equivalents (lithium enolate, aza enolate, silyl enol ether,

enamines, zinc enolates), Knoevenagel reaction, Perkin

reaction, Reformatsky reactions, Cannizaro reaction, Mannich

reaction intramolecular aldol reaction Ac lation Claisen

condensation, Dieckman cyclization, Conjugate addition of

enolates, Robinsons Annulation

• carbonyl compounds, chemoselective reduction of carbonyl

compounds.

• e rosyn es s : n ro uc on o re rosyn e c ana ys s

• Applications: to solve problems and design routes for synthesis of complex molecules



Text BooksText Books

Main Text Book: J. Clayden, N. Greeves, S. Warren and P.

o ers, rgan c em s ry , rs ., x or n vers y ress,

2001.

K. P. C. Volhardt and N. E. Schore “Organic Chemistry: Structure and

Function”, 5th Ed., W. H. Freeman and Company, 2007.

L. G. Wade Jr. “Organic Chemistry”. 7th Ed., Prentice Hall, 2009.

T. W. G. Solomons, C.G. Fryhle. “Organic Chemistry”, 9th Ed., Wiley-

India, 2008.

F. A. Carey and R. J. Sundburg, “ Advanced Organic Chemistry, Part A&B”, 5th Ed., Plenum Press, 2007.

Advanced Organic Chemistry : Jerry March

ereoc em s ry o ar on ompoun s: . . e



• • Marks for attendance

– XX will be awardedif you miss more than

• < 95% = 4• < 90 % = 3

9 lectures • < 85% = 2

• < 80% = 1

• Quiz 10 x 2

• Midsem 30

• Endsem 46

• Attendance 4



• No late entries in class, pl note the time (11.05 sharp)

Important points before we start

• Will be marked absent i f you arrive after 10 mins

• Cell phones should be on silence mode

• , , ,

• You may ask questions, preferably about the topic• I may not know the answer

• time constraints

• we will find out together

• Open to suggestions – feed back form, constructive

criticism is welcome

• My observation/views on –• General ‘attitude’,

•

Lets have a round of intro

,

• Reco letters,• Copying in any form



•

4,5 & 6 (relevant



The Layout of Periodic TableThe Layout of Periodic Table

Lecture slides are courtesy of Oxford University Press



The Organic Chemist’s Periodic TableThe Organic Chemist’s Periodic Table

Organic Chemist’s language1. Structure and representation of a molecule (static)

2. Description of reaction mechanism (dynamic) (curly arrows)

Why molecules react/ don’t react with each other?



Organic reaction

• •

molecules

molecules with

dipoles

• Possess the

re uired activation• Orbital overlap

energy• Flow of electrons

• From Nucleophiles to Electrophiles



Why organic reactions are slow ??



Organic Reaction

• Interaction between HOMO of a nucleophile and LUMO of

an electro hile leads to a reaction.

• The two partners should be matched with respect to charge-charge attraction and energy and orientation of orbitals

nvo ve

• Nu- may donate e- from a lone pair (e.g. NH2, OH), a π bond

or a si ma bond in reference

• E+ may accept e- into an empty orbital or an antibonding

orbital of a π bond or even a sigma bond in the above

pre erence or er • These antibonding orbitals are of low enough energy if the

bond is olarized e. CO or if it is weak Br

• Nu- (OH, NH2

), E+ (CO, RX), hydrocarbons - not reactive.



OrbitalsOrbitals ofof NucleophilesNucleophiles andand ElectrophilesElectrophiles



Filled Nonbonding + Empty NonbondingFilled Nonbonding + Empty Nonbonding



Filled Nonbonding +Filled Nonbonding + SigmaSigma Antibonding Antibonding



Filled Nonbonding +Filled Nonbonding + PiPi Antibonding Antibonding



Pi bonding +Pi bonding + Empty NonbondingEmpty Nonbonding



Filled pi as nucleophile and sigma star of Br 2 is electrophile



Comparison of C=C & C=OComparison of C=C & C=O ππ bondsbonds

e = on s t e most mportant unct ona group n organ c c em stry.

Present in aldehydes, ketones, acids, esters, amides and so on.



The Carbonyl (C=O) GroupThe Carbonyl (C=O) Group

End-on overlap Side-on overlap



NucleophilicNucleophilic Addition to C=O Addition to C=O



MOs Involved in theMOs Involved in the NucleophilicNucleophilic Addition to C=O Addition to C=O

Electrostatic attraction

& low energy π* orbital

HOMO

Very stable π orbital



NucleophilicNucleophilic Addition to C=O Addition to C=O

A curly arrow means actual movement of a pair of electrons

from a filled orbital into an empty orbital

Here the Nu still attacks the C and not the O. This is because of the orbital overlap:

The pi star orbital has a larger lobe is on C and not on O.

The positive charge is neutralized anyway.



MO Snap ShotsMO Snap Shots



Example:Example: CyanohydrinCyanohydrin FormationFormation



BurgiBurgi--DunitzDunitz TrajectoryTrajectory

Why aldehydes react faster than ketones?



Br is an electro hile or a nucleo hile?

There is no dipole at all!!



Large HOMOLarge HOMO--LUMO Gap Makes Strong BondLUMO Gap Makes Strong Bond

Bromine is strongly electrophillic !!

Weak sigma bonds Strong sigma bonds

Low LUMO and high HOMO

Low HOMO and High LUMO



Organic Reaction

• Interaction between HOMO of a nucleophile and LUMO of

an electro hile leads to a reaction.

• The two partners should be matched with respect to charge-charge attraction and energy and orientation of orbitals

nvo ve

• Nu- may donate e- from a lone pair (e.g. NH2, OH), a π bond

or a si ma bond in reference

• E+ may accept e- into an empty orbital or an antibonding

orbital of a π bond or even a sigma bond in the above

pre erence or er • These antibonding orbitals are of low enough energy if the

bond is olarized e. CO or if it is weak Br

• Nu- (OH, NH2), E+ (CO, RX), hydrocarbons - not reactive.



HydrideHydride NucleophileNucleophile??

Can we use NaH?



MO PictureMO Picture

Reaction is called as ‘hydride transfer’

It is the BH sigma bond !!!



Problems

We discussed CN- and hydride transfer

We will discuss organometallics in next lectureLets talk about some neutral Nucleophiles such as water and alcohol



HydrationHydration



Formaldehyde is extremely reactive and it is prone to polymerization

No steric hindrance for going from sp2 to sp3



Extent of hydration

Governing factors

Steric hindrance



Write mechanism for formation of ketal of acetone or acetonide of ethylene glycol.



Cyclopropanone exists as the hydrate in water but

2-hydroxyethanal does not exists as hemiacetal.

Explain why?



Write down the structures of all the possible products of NaBH4 reduction.

How would you distinguish between them?

Which of the three carbonyls will be hydrated?



Equilibrium with aldehyde



Predict the roduct and su est mechanism

IR - No peak between 1600-1800 Cm-1But shows peak betn. 3000-3400 cm-1

CMR

no peak above 150 but shows a peak at 110 ppm

Explain



Su est ste s with rea ents and mechanism

O OH

lecture 1-2 suvarn_addition to c=o, mo aspects

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