Organic Chemistry -- Analyzing Organic Reactions

Lewis acids

- electron acceptors

- have vacant orbitals or positively polarized atoms

Lewis bases

- electron donors

- have a lone pair of electrons and are often anions

Bronsted-Lowry acids

proton donors

Bronsted-Lowry bases

proton acceptors

Amphoteric molecules

molecules with the ability to act as either Bronsted-Lowry acids or bases depending on the reaction

Ka

Acid dissociation constant. A measure of acidity. It is the equilibrium constant corresponding to the dissociation of an acid, HA, into a proton and its conjugate base.

pKa

-logKa

A lower pKa indicates a

stronger acid

How does pKa change with the periodic table?

decreases down the periodic table and increases with electronegativity

Alpha-hydrogen

hydrogen atoms connected to alpha-carbons adjacent to carbonyls

Alpha-hydrogens are (acidic/basic)

acidic

What are common acidic functional groups?

alcohols, aldehydes, ketones, carboxylic acids, carboxylic acid derivatives

What are common basic functional groups?

amines and amides

Nucleophiles

-nucleus loving

-negatively charged so they are attracted to the + nucleus

-have lone pairs or π bonds that can be used to form covalent bonds to electrophiles

Nucleophiles have _____ electron density and often have a ______ charge

increased; negative

Nucleophlilicity is similar to _______; however, nucleophilicity is a ______ property while basicity is _________

basicity; kinetic; thermodynamic

What properties can affect nucleophilicity?

charge, electronegativity, steric hindrance, and the solvent

What groups are common organic nucleophiles?

amino groups

Electrophiles

"Electron-loving" atoms with a positive charge or positive polarization; can accept an electron pair when forming new bonds with a nucleophile.

More ______ compounds are more electrophilic

positive

What groups can act as electrophiles?

alcohols, aldehydes, ketones, carboxylic acids, and their derivatives

Leaving groups

-molecular fragments that retain electrons after heterolysis

-Weak bases are more stable with extra electrons -> good leaving groups

Heterolysis

cleavage of a bond in which both electrons are given to the same atom

The best leaving groups can

stabilize additional charge through resonance or induction

Weak bases

Substances capable of accepting hydrogen but do not completely ionize in solution

What types of molecules make a good leaving group?

bases

What type of molecules are almost never leaving groups?

alkanes and hydrogen ions (because they form reactive anions)

Unimolecular nucleophilic substitution (SN1) reactions

- leaving group leaves in the first step and forms a carbocation

- nucleophile attacks the planar carbocation in the second step from either side, leading to a racemic mixture of products

What types of molecules do SN1 reactions prefer and why?

more substituted carbons because the alkyl groups can donate electron density and stabilize the positive charge of the carbocation

The rate of an SN1 reaction is dependent on

only the concentration of the substrate: rate = k[R-L], where L is the leaving group and R is the rest of the molecule

Bimolecular nucleophilic substitution (SN2) reactions

- nucleophile performs a backside attack at the same time as the leaving group leaves

What happens to the stereochemistry of the molecule in an SN2 reaction?

inverts due to the backside attack of the nucleophile; changes the absolute configuration from R to S and vice versa if the incoming nucleophile and the leaving group have the same priority in the molecule

What type of molecules do SN2 reactions prefer and why?

less substituted carbons because the alkyl groups create steric hindrance and inhibit the nucleophile from accessing the electrophilic substrate carbon

The rate of an SN2 reaction is dependent on

the concentrations of both the substrate and the nucleophile: rate = k[Nu:][R-L], where Nu: is the nucleophile, L is the leaving group, and R is the rest of the molecule

Oxidation state

the charge of an atom that it would have if all its bonds were completely ionic

Which carbon-based molecule has the lowest oxidation state (most reduced)?

CH4

Which carbon-based molecule has the highest oxidation state (most oxidized)?

CO2

Which functional groups are the most oxidized?

carboxylic acids and their derivatives; followed by aldehydes, ketones, and imines; followed by alcohols, alkyl halides, and amines

Oxidation

loss of electrons

Oxidizing agents

- accept electrons and are reduced in the process

- have high affinity for electrons

- have unusually high oxidation state

- often contain a metal and a large number of oxygens

Alcohol --> Aldehyde/Ketone reagents

PCC, CrO3/pyridine

Aldehyde --> Carboxylic acid reagents

H2CrO4, KMnO4, H2O2

Alcohol --> Carboxylic acid reagents

KMnO4, H2CrO4

Alkane --> Carboxylic acid reagents

KMnO4

Alkene --> Aldehyde + Ketone reagents

O3, then Zn or O3, then CH3SCH3

Alkene --> Carboxylic Acid + Ketone reagents

1. O3, then H2O2

2. KMnO4, heat, H3O+

Alkyne --> Carboxylic Acid reagents

O3, then H2O2; KMnO4, heat, H3O+ (same as Alkene to Carb.Ac. & Ketone)

Alkene --> Vicinal Diol reagents

OsO4; KMnO4, HO-

Alkene --> Epoxide reagents

mCPBA

Diol --> Aldehyde reagents

1. NaIO4

2. Pb(OAc)4

3. HIO4

Ketone --> Ester reagents

1. mCPBA (same as Alkene/Epoxide)

Reduction

gain of electrons (decrease in oxidation)

Reducing agents

-elements in redox reactions that donate electrons to another species and are oxidized in the process

-it loses electrons so it becomes oxidized while reducing other species

-contain metals bonded to a large number of hydrides

Aldehydes, ketones, and carboxylic acids can be reduced to

alcohols

Amides can be reduced to

amines

Esters can be reduced to

a pair of alcohols by LiAlH4

Both nucleophile-electrophile and oxidation-reduction reactions tend to act at

the highest priority (or most oxidized) functional groups

What are often used as protecting groups for aldehyde or ketone carbonyls?

diols