Orgo I Review Flashcards
Type of Carbon Bonds
Carbon forms four bonds:
sp3 (four single bonds)
sp2 (double bond)
sp (triple bond)
%s Character
The greater the %s character, the more electronegative is the atom.
Alkyl Groups
Alkyl groups are electron-releasing (ERGs) both inductively (sp3 hypridized and less
electronegative) and via hyperconjugation.
Acid vs Base: H
Every H is potentially acidic.
Base Make-Up
A base has a nonbonding pair of electrons: a negative ion, a nitrogen atom, an oxygen atom
(and atoms in the same group).
Strong Acid vs. Weak Base
The stronger acid has the weaker conjugate base.
Weaker Base Stability
The weaker base is the more stable base.
Stabilizing factors include: electronegativity
(across a row), size (down a column), resonance (charge delocalization), and an EWG
stabilizing the negative charge.
Nucleophile
A nucleophile has a nonbonding pair of electrons.
Nucleophilicity
Nucleophilicity increases with basicity for the same nucleophilic atom and atoms in the same
row.
Nucleophilicity increases with size because of increasing polarizability and
decreasing solvation.
The better leaving group is the weaker base.
Predicting Reactions
An alkyl halide or tosylate and protonated alcohols contain a leaving group.
Consider the reaction conditions and ask:
1) will the leaving group leave (will a carbocation form)?
2) is a strong base present (E2)?
3) is a good nucleophile present (SN2)?
If a carbocation forms it can:
1) react with a nucleophile (SN1)
2) lose a β-H to form an alkene (E1)
3) rearrange to a more stable one
Alkenes undergo electrophilic addition. The electrophile adds to give the most stable
carbocation.
Alkenes & HBr
Alkenes undergo free radical addition with HBr with peroxides or light. The attacking atom is a
bromine that adds to give the most stable free radical.
Hydroboration-oxidation
syn, anti-Markovnikov addition of water
Epoxidation
syn
Ozonolysis-reductive workup
cleaves C=C to give aldehydes and ketones
Partial reduction of alkynes
Lindlar Pd, Na/NH3
Free radical halogenation of alkanes
photochemical chlorination/bromination; both statistical
and relative reactivity effects must be considered
Stereochemistry
stereoisomers, E/Z, cis/trans, enantiomers, R/S, diastereomers, meso
compounds, optical activity, specific rotation, optical purity = enantiomeric excess (ee)
Substituent effects
inductive and resonance effects; EWGs and ERGs; an atom with a nonbonding
pair of electrons is strongly electron-releasing via resonance
Acid-Base Rxns: Strong Base
Since acid-base reactions are rapid and reversible, always check to see if a strong base is present. If there is a base, look for an acidic H.
Relative Acidity
In discussing relative acidity, look at the conjugate base and try to assess the stability.
Stability of a Negative Charge
The stability of a negative charge is usually governed by electronegativity when comparing atoms in a row and size when comparing atoms in a column.
E2 Rxn of Alkyl Halide with a Strong Base
The E2 reaction of an alkyl halide with a strong base occurs through an anti-periplanar transition state (the leaving groups are anti) and the more stable alkene (Zaitsev) predominates.
In a cyclohexane chair conformation, the leaving groups must be axial.
SN2: Steric Hindrance
The SN2 reaction is subject to steric hindrance and occurs with inversion of configuration.
Carbocation Stability: Solvent Polarity
Carbocation stability is particularly sensitive to solvent polarity. Water is the best of the common solvents for
separating and solvating ions.
Stabilizing influences include resonance delocalization, inductive effects, and electronegativity.
Destabilizing factors include steric hindrance, inductive effects, and electronegativity.
Neclophilicity Trends
In comparing atoms in a row, nucleophilicity increases with basicity; in a column, nucleophilicity increases with size.
Better Leaving Group
The better leaving group is the weaker base.
ERG & EWG
Groups of atoms (functional groups) can be characterized as electron-releasing (ERG) or electron-withdr.awing (EWG).
Alkyl groups are ERGs by both the inductive (less EN carbon due to lower %-s character) and resonance (hyperconjugation) effects.
Oxygen (and N!) substituents are strongly ERG since the resonance effect outweighs the inductive effect.
Heat of Formation
The heat of formation is the energy involved in forming one mole of a compound from the elements in their most stable form at 25°C. The heat of formation of a compound is the energy evolved when one mole of the compound is burned to CO2 and water.
Branching
Branching is stabilizing probably due to intramolecular dispersion forces.
Ring Strain
Ring strain is the difference between the calculated ΔHf or ΔHc and the actual ΔHf (or ΔHc). Contributors to ring strain include: angle strain, torsional strain, and steric hindrance.
Axial Substituents
Axial substituents are destabilized by 1,3-diaxial interactions.
Cyclohexane Chair Conformations
There are two chair conformations for any cyclohexane species and the most stable one has the fewest axial substituents or has the largest substituent equatorial.
Conformations
Conformations are spatial orientations that can be interconverted by rotation around single bonds.
Enantiomers
Enantiomers are nonsuperimposable mirror images.
Diastereomers
Diastereomers are stereoisomers that are not enantiomers.
Meso Compound
A meso compound is an optically inactive (achiral-plane of symmetry) form of a compound that can be optically active.
Most Stable Conformation of Cyclohexanes
The most stable conformation of cyclohexanes is the chair and the most stable conformation of substituted cyclohexanes is the one with the largest substituent in the equatorial position. Analyze the cis and trans stereoisomers using the relative orientations of “up” and “down.”
Carbocations can…
Carbocations can
1) react with nucleophiles (SN1);
2) lose β-hydrogens to give alkenes;
and 3) rearrange to a more stable one.
Solvolysis
Solvolysis reactions are unimolecular and give carbocations.
Hydroboration-oxidation
Hydroboration-oxidation results in anti-Markovnikov addition of water to a double bond; the addition is syn.
Addition of Halogens to Alkenes
Addition of halogens to alkenes gives a vicinal dihalide and the addition is anti via a halonium ion.
Solvation of a Nucleophile
Solvation of a nucleophile reduces its nucleophilicty.
Smaller ions are better solvated than larger ones.
Lewis Bases, Bronsted Bases & Nucleophiles
Lewis bases, Bronsted bases, and nucleophiles are terms for the same species: species that contain nonbonding electrons.
E2 Rxns: Size of Base
E2 reactions are affected by the size of the base.
Typical NaOR bases give Zaitsev product distributions; bulky bases give Hofmann product distributions.
Nucleophiles & Carbonyl Carbon
Nucleophiles attack the carbonyl carbon leading to a tetrahedral intermediate. With aldehydes and ketones, protonation of the alkoxide leads to nucleophilic addition. With carboxylic acid derivatives, the tetrahedral intermediate collapses with the loss of the best leaving group to give a more stabilized carboxylic acid derivative. Hemiacetals and hemiaminals undergo loss of water to give stabilized carbocations, which react further (alcohols give acetals; primary amines give imines; secondary amines give enamines).
