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7/28/2019 MCAT+OC2+Hybrid+ +Handout

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IR

Infrared Spectroscopy (IR) measures the frequency at which covalent bonds bend, stretch, and rotatewhen the molecule is irradiated with light in the infrared region.

SpectroscopySpectroscopy

IR spectroscopy is primarily used on the MCAT to identify the specific functional groups that a

molecule contains.

IR defined

p. 248

What you need to know:

- 3200 3600 cm-1: OH (broad) and NH (sharp)

- 1680 1750 cm-1: C=O (strong)


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NMR

Nuclear Magnetic Resonance (NMR) exploits the inherent spin of odd-numbered nuclei. Whensuch a sample is placed in an applied magnetic field, nuclei in the lower energy state are promoted to ahigher energy state. NMR measures the frequency of this energy difference.

What you need to know about proton NMR:

sp3sp2

C

C~2.5 ppm

10 8 6 4.5 0

ppm

CC C C

H HHH

O O

downfield

(deshielded)

H

H

p. 249


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NMR

sp3

sp

sp2

0 - 4.5 ppm peaks; hydrogen retains almost full electron cloud.

2 - 4 ppm peaks; shielded hydrogens share some electron cloud.

4.5 -10 ppm peaks; deshielded hydrogens have smaller electron cloud.


aromatic

ewgs

aldehydic

p. 249

6 - 8 ppm peaks; from hydrogen electron clouds on aromatic sp2carbons .

1-2 shift in ppm; electron withdrawing groups pull downfield.

9-10 ppm peaks; carboxylic groups further shifted (10-12 ppm).


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Aldehyde/Ketone common namesNaming Convention

2-propanone

acetone

benzaldehyde

benzaldehyde

Aldehydes and KetonesAldehydes and Ketones

IUPAC name:Common name:


p. 250

ethanalacetaldehyde

methanalformaldehyde

Directed Practice

Organic Chemistry Review Notes: Chapter 8 (Aldehydes and Ketones)




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Key Reaction

Because the carbonyl carbon is an electrophile, a key reaction of aldehydes and ketonesinvolves nucleophilic addition by a strong nucleophile.

p. 252

a tetrahedral

intermediate

a strong nucleophile!

strong nucleophiles:

- OH, OR, CN, H, RMgX (R)

weak nucleophiles:

- H2O, ROH, NH3 (neutral!)

- need an acid catalyst (usually H

+

)


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Critical Thinking Exercise

What question should you always ask yourself after an attack on a carbonyl?

Ask, Do I have a leaving group? (Can I re-form the carbonyl?)


Question: How do you recognize a leaving group?

Answer: By what it is not!

- a hydrogen never leaves.

- a carbon never leaves.

- ever thin else can. Usual LGs are . . .

CTE: reform the carbonyl

p. 252

Restoring the carbonyl group gives a more stable (favored) final state. Adding an external H+ is still possible, but a carbonyl will

always be preferred.

The takeaway:

- only aldehydes and ketones undergo addition to the carbonyl.

- all other carbonyl-containing compounds undergo substitution.

(a) -S

(b) -O

(c) -N

lousy LG

decent LG

best LG of the three


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Acid-catalyzed formation of a Hemiketal from a ketone


a weak nucleophile!

Td

(I)

(II)

(III) (IV)

p. 252


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Acid-catalyzed reaction of hemiacetals to make acetals


H+

(I)(II)

(III)

(IV)

H

p. 252

Directed Practice

Organic Chemistry Review Notes: Chapter 8 (Aldehydes and Ketones)

H+

(IIIa)


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Wolff-Kishner Reduction of a carbonyl to make an alkane


Also:Zn(Hg)

in base

in acid

conc. HCl

Wolf-Kishner and Wittig Reaction

p. 253

Wittig Reaction of carbonyl with phosphonium ylide to make an alkene

This is the Clemmenson reduction.

R1

R4R2

R3

Note: your Lesson Book

should have this!

R1

R2

R3

R4

:

+


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Nomenclature

Common Examples

2-hydroxypropanoic acid (lactic acid)

2-methylpropenoic acid

Carboxylic AcidsCarboxylic Acids

Carboxylic Acid common names

p. 255

cyclohexanecarboxylic acid

ethanedioic acid

(oxalic acid)

2-butyl-heptanoic acid

*


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Critical Thinking Exercise

Which is more acidic, Cl3CCOOH or ClCH2COOH?

Which is more acidic, FCH2COOH or CH3OCH2COOH?

Which is more acidic, FCH2CH2COOH or CH3CHFCOOH?

More Cl groups => better induction. This is a # effect.

F is more electronegative than O => better induction. This is an EN effect.


CTE: induction effects

p. 256

Closer F group => better induction. This is a proximity effect.

The mantra:

electron-density withdrawing groups make acids stronger.

*

How do you recognize an electron withdrawing group?

- it is an EAS deactivator (halogen / nitro / carbonyl / cyano).

Question:


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Synthesis

Oxidative cleavage of alkenes

The oxidative cleavage of alkenes also produces carboxylic acids.


Oxidative cleavage, alkenesp. 258

KMnO4

KMnO4,


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Naming chart/rulesNomenclature

Derivative Structure Name of Derivative

Acid chloride

Acid

ethanoyl chloride

benzoic anh dride

Carboxylic Acid DerivativesCarboxylic Acid Derivatives

*

p. 259Directed Practice Organic Chemistry Review Notes: Chapter 10 (Carboxylic Acid Derivatives)

Ester

Amide

isopropyl propanoate

N-benzyl ethanamide


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Order of Reactivity

Rank the following carboxylic acid derivatives in order of reactivity.

Most reactive

Acyl halide1

Acid anhydride

3a3b

Carboxylic Acid DerivativesCarboxylic Acid Derivatives

LG Conjugate

Acid: pKa

HX : 6

RCOOH: ~5

*

Reactivity of derivatives

Least reactive

p. 261

2Ester Carboxylic acid

Amide

4ROH/HOH : ~16

R2N-H : ~33

Takeaway:

The stronger the conjugate acid

of the LG {the more () the pKa},

the more reactive the acyl derivative!


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Aldol Condensation

Step 1: aldehyde enolate

Step 2: enolate attacks

a second aldeh de molecule

Carbonyl Compounds as NucleophilesCarbonyl Compounds as Nucleophiles

Aldol Condensationp. 263

Step 3: protonation . . .

. . . if heat, then

Step 4: dehydration

Td

because theres no LG

aldol, unless . . .


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The Claisen condensation reaction

Carbonyl Compounds as NucleophilesCarbonyl Compounds as Nucleophiles

Td, with LG

Claisen condensationp. 264

Similar mechanism: enolate attacks a carbonyl carbon, but different reaction.

Aldol rxn = net addition. Claisen rxn = net substitution.

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