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Organic Chemistry I: Reactions and Overview
Andrew Rosen
- January 13, Editor: Raghav Malik
- I Library of Synthetic Reactions Contents
- II Organic Trends and Essentials
- 1 The Basics: Bonding and Molecular Structure
- 2 Families of Carbon Compounds
- 2.1 Strength of London Dispersion Forces (Polarizability)
- 2.2 Degree of Unsaturation
- 3 An Introduction to Organic Reactions and Their Mechanisms
- 3.1 Comparing Acid Strengths
- 4 Nomenclature and Conformations of Alkanes and Cycloalkanes
- 5 Stereochemistry
- 5.1 Naming Enantiomers via the -R and -S System
- 5.2 Stereochemistry Examples
- 6 Ionic Reactions - Overview
- 6.1 General Nucleophilic Substitution Reactions
- 6.2 Carbocation Stability
- 6.3 Factors Aecting the Rates of SN 1 and SN 2 Reactions
- 6.4 Elimination Reactions
- 6.5 Summary
- 7 Alkenes and Alkynes I - Overview
- 7.1 The E-Z System
- 7.2 Relative Stabilities of Alkenes
- 7.3 Factors Aecting Elimination Reactions
- 7.4 Acid-Catalyzed Dehydration of Alcohols
- III Reaction Mechanisms
- 8 Ionic Reactions - Mechanisms
- 8.1 The SN 2 Reaction
- 8.2 The SN 1 Reaction
- 8.3 The E2 Reaction
- 8.4 The E1 Reaction
- 9 Alkenes and Alkynes I - Mechanisms
- 9.1 Acid-Catalyzed Dehydration of Secondary or Tertiary Alcohols: An E1 Reaction
- 9.2 Acid-Catalyzed Dehydration of Primary Alcohols: An E2 Reaction
- 9.3 Synthesis of Alkynes from Vic-Dihalides
- 9.4 Substitution of the Acetylenic Hydrogen Atom of a Terminal Alkyne
- 9.5 Deprotonation Reagents
- 9.6 Hydrogenation
- 10 Alkenes and Alkynes II - Mechanisms
- 10.1 Addition of H−X to an Alkene
- 10.2 Acid-Catalyzed Hydration of an Alkene
- 10.3 Mercuration-Demercuration and Hydroboration-Oxidation
- 10.4 Summary of H−X and H−OH Additions
- 10.5 Electrophilic Addition of Bromine and Chlorine to Alkenes
- 10.6 Halohydrin Formation from an Alkene
- 10.7 Oxidative Cleavage of Alkenes
- 10.8 OsO 4 Reaction
- 10.9 Summary for Dihalide, Dihydroxy, and Carbene Additions
- 10.10 Electrophilic Addition of Bromine and Chlorine to Alkynes
- 10.11 Addition of Hydrogen Halides to Alkynes
- 10.12 Oxidative Cleavage of Alkynes
- 11 Alcohols and Ethers - Mechanisms
- 11.1 Alcohols with H−X
- 11.2 Alcohols with PBr 3 or SOCl
- 11.3 Leaving Group Derivatives of Alcohols
- 11.4 Converting OH to LG Summary
- 11.5 Synthesis of Ethers
- 11.6 Protecting Groups
- 11.7 Ether Reactions Summary
- 11.8 Epoxides
- 11.9 Epoxide Reaction Summary with Example
- 12 Alcohols from Carbonyl Compounds - Mechanisms
- 12.1 Alcohols by Reduction of Carbonyl Compounds
- 12.2 Oxidation of Alcohols
- 12.3 Alcohols from Grignard Reagents
- 13 Radical Reactions - Mechanisms
- 13.1 Bromination
- 13.2 Chlorination
Part II
Organic Trends and Essentials
1 The Basics: Bonding and Molecular Structure
1.1 Resonance Stability
- The more covalent bonds a structure has, the more stable it is
- Charge separation (formal charges) decreases stability
- Negative charges on the more electronegative elements and positive charges on the more electropositive elements are more favorable^2
2 Families of Carbon Compounds
2.1 Strength of London Dispersion Forces (Polarizability)
- Large atoms are easily polarizable and small atoms are not
- Atoms with unshared electron pairs are more polarizable than atoms with only bonding pairs
- Molecules that are longer and atter (long chains) have more surface area and thus have larger dispersion forces when other factors are similar
2.2 Degree of Unsaturation
- A degree of unsaturation is either a π bond or a ring structure
- Formula:
2 C + 2 + N − H − X
where the variables are the number of carbons (C), nitrogens (N), hydrogens (H), and halogens (X)
3 An Introduction to Organic Reactions and Their Mechanisms
3.1 Comparing Acid Strengths
Factors Aecting Acidity (in decreasing signi cance)^3 : ARIO
- Atom
- Resonance Stabilization
- Induction Eect
- Orbital (s character)
(^2) For the purposes of drawing all resonance structures, it is not considered a violation of the octet rule if a second-row element, like carbon, has fewer than an octet. It is less likely but still imperative to draw. (^3) This general trend is not always perfectly applicable. However, it is usually a fairly good indicator.
ARIO Explained:
- Atom: Look at what atom the charge is on for the conjugate base.
For atoms in the same row, we consider electronegativity. The further to the right on the periodic table an atom is, the more electronegative it is. If a conjugate base's negative charge is on a more electronegative atom, it is more stable, and thus the parent acid is stronger. For atoms in the same column, we consider an atom's ability to stabilize a charge. The further down on the periodic table an atom is, the better it is at stabilizing a charge. If a conjugate base's negative charge is more stabilized on an atom further down a group, it is a more stable molecule, and thus the parent acid is stronger.
- Resonance Stabilization: Look at resonance structures. The more distributed the charge of the conjugate base is, the stronger the parent acid is.
- Inductive Eect: Look for inductive eect. If there are many electronegative atoms near the conjugate base's negative charge, electron density is pulled toward these atoms. This creates more stable anions and thus more acidic parent molecules. However, if there are many alkyl groups, this is a process called hyperconjugation, and the parent acid is actually less stable.
- Orbital: Look at the orbital where the negative charge for the conjugate base is. More s character of a bond with hydrogen makes it more acidic.
4 Nomenclature and Conformations of Alkanes and Cycloalkanes
4.1 Ring Flipping
- The axial groups become equatorial and vice versa
When doing a ring ip, whether a group is up or down does not change
- Chair Conformation 1: Chair Conformation 2 (after ring ip):
- When performing a chair ip, each atom is rotated one spot in the clockwise direction
- A molecule is more stable when steric hindrance is minimized and bulky substituents are equatorial as opposed to axial
5 Stereochemistry
5.1 Naming Enantiomers via the -R and -S System
- Each of the four groups attached to the chirality center is assigned a priority of 1, 2, 3, or 4. Priority is assigned on the basis of the atomic number of the atom that is directly attached to the chirality center. The group with the highest atomic number gets the highest priority and vice versa.
- When a priority cannot be assigned on the basis of atomic number of the atoms, then the next set of atoms in the unassigned groups is examined. This process is continued until a decision can be made at the rst point of dierence.
- The rates of SN 2 reactions (not SN 1 ) depend on both the concentration and identity of the attacking nucleophile
- In a selection of nucleophiles in which the nucleophilic atom is the same, nucleophilicities parallel basicities:
RO−^ > HO−^ RCO− 2 > ROH > H 2 O
- Nucleophiles parallel basicity when comparing atoms in the same period
- Nucleophiles do not parallel basicity and, instead, parallel size when comparing atoms of the same group
- The best leaving groups are weak bases after they depart
- Polar aprotic solvents favor SN 2 and polar protic solvents favor SN 1
Most of the solvents with abbreviated names are polar aprotic
6.4 Elimination Reactions
- Higher temperatures increase the rates of elimination reactions
- A product with a more substituted double bond is more stable and thus more favorable
- If tert-butoxide is used, sterics must be considered to nd out which hydrogen it takes through the E 2 reaction
6.5 Summary
- Note: It is debatable, but secondary molecules can have SN 1 or E 1 in polar protic solvents
7 Alkenes and Alkynes I - Overview
7.1 The E-Z System
- To determine E or Z, look at the two groups attached to one carbon atom of the double bond. Decide which has higher priority. Then, repeat this at the other carbon atom.
If the two groups of higher priority are on the same side of the double bond, the alkene is designated Z. If the two groups of higher priority are on opposite sides of the double bond, the alkene is designated E.
7.2 Relative Stabilities of Alkenes
- The trans isomer is generally more stable than the cis isomer
- The greater number of attached alkyl groups, the greater the stability of an alkene
7.3 Factors Aecting Elimination Reactions
- A non-bulky base favors the more substituted double bond while a bulky base favors in making the less substituted double bond
7.4 Acid-Catalyzed Dehydration of Alcohols
- Rearrangements, also known as 1,2 shifts, can occur in primary and secondary alcohol dehydration
The more favored product is dictated by the stability of the alkene being formed
- For dehydration of secondary alcohols, the positive charge is shifted through a hydride shift or alkyl shift
- For the dehydration of primary alcohols, a carbocation is not formed as an intermediate. However, rearrangements can still occur after dehydration. The resulting alkene's π bond is broken when a hydrogen atom from the acid bonds to the carbon to form a carbocation. Rearrangement then occurs as usual.
- A ring can change in size due to a methyl shift, especially to reduce ring strain. An example is shown below:
- Note: Never do two migrations
8.2 The SN 1 Reaction
- An SN 1 reaction will cause racemization if enantiomers are possible products
8.3 The E2 Reaction
- There must be an anti-coplanar nature
8.4 The E1 Reaction
- E1 reactions almost always accompany SN 1 reactions to some extent
9 Alkenes and Alkynes I - Mechanisms
9.1 Acid-Catalyzed Dehydration of Secondary or Tertiary Alcohols: An E1 Reaction
9.6 Hydrogenation
- Metal catalyzed H 2 addition to an alkyne (eg: H 2 /Pd-C) produces an alkane
- Controlled metal catalyzed H 2 addition to an alkyne (eg: H 2 and Lindlar's Catalyst^5 ) produces an alkene with syn- addition (cis). This is also for H 2 /Ni 2 B(P−2)
- Chemical reduction of an alkyne produces an alkene with anti-addition (trans). Sodium metal and liquid NH 3 is one example. Another is Li, C 2 H 5 with NH 4 Cl
10 Alkenes and Alkynes II - Mechanisms
10.1 Addition of H−X to an Alkene
- Markovnikov, not stereospeci c, and rearrangements are possible
10.2 Acid-Catalyzed Hydration of an Alkene
- Markovnikov, not stereospeci c, and rearrangements are possible
10.3 Mercuration-Demercuration and Hydroboration-Oxidation
- Mercuration-Demercuration: Markovnikov addition, anti stereochemistry, and no rearrangements
Uses Hg(OAc) 2 , H 2 O and then NaBH 4 , NaOH
- Hydroboration-Oxidation: Anti-Markovnikov addition, syn stereochemistry, and no rearrangements
Uses BH 3 and then H 2 O 2 , NaOH
(^5) Lindlar's Catalyst is Pd/CaCO 3 /Pb
10.4 Summary of H−X and H−OH Additions
10.5 Electrophilic Addition of Bromine and Chlorine to Alkenes
10.9 Summary for Dihalide, Dihydroxy, and Carbene Additions
10.10 Electrophilic Addition of Bromine and Chlorine to Alkynes
- Alkynes show the same kind of halo-addition as alkenes (anti-addition)
- Addition may occur once or twice depending upon the molar equivalents of the halogen reagent
10.11 Addition of Hydrogen Halides to Alkynes
- Alkynes react with one molar equivalent of HX to form haloalkenes and with two molar equivalents to form geminal dihalides via Markovnikov's Rule
- Anti-Markovnikov addition occurs when peroxides are present
10.12 Oxidative Cleavage of Alkynes
- Oxidative cleavage of alkynes with ozone will yield two carboxylic acids
11 Alcohols and Ethers - Mechanisms
11.1 Alcohols with H−X
- Racemic mixtures are produced if enantiomers are possible
- Rearrangements are present
- Methanol and 1 ◦^ alcohols go through an SN 2 mechanism. 2 ◦^ and 3 ◦^ alcohols go through an SN 1 mechanism
11.2 Alcohols with PBr 3 or SOCl 2
- Converts a 1 ◦^ or 2 ◦^ alcohol to a leaving group without rearrangements
- Inversion of con guration occurs since the reaction is SN 2
11.3 Leaving Group Derivatives of Alcohols
- Using either pyridine or DMAP, sulfonate esters can be prepared from combining an alcohol with a chlorinated sulfonate derivative
- There is retention of con guration with this reaction
11.4 Converting OH to LG Summary
11.5 Synthesis of Ethers
- Alcohols can dehydrate to form alkenes, as mentioned in Section 7. Also, 1 ◦^ alcohols can dehydrate to form ethers by the following mechanism:
- Acid-catalyzed dehydration is not useful for preparing unsymmetrical ethers from dierent 1 ◦^ alcohols because the reaction leads to a mixture of products (ROR, ROR′, and R′OR′)
- Alkoxymercuration-demercuration is a method for synthesizing ethers directly from alkenes, like in the example below, and parallels oxymercuration-demercuration
11.9 Epoxide Reaction Summary with Example
Example of BLAM:
12 Alcohols from Carbonyl Compounds - Mechanisms
12.1 Alcohols by Reduction of Carbonyl Compounds
- Reduction converts a carboxylic acid to a primary alcohol by taking o an oxygen from C−−O
- Reduction converts an ester into two 1 ◦^ alcohols, one derived from the carbonyl part of the ester group and the other from the alkoxyl part of the ester
- Reduction converts a ketone to a 2 ◦^ alcohol and an aldehyde to a 1 ◦^ alcohol
- Aldehydes and ketones are easily reduced by NaBH 4. LAH is another reducing agent:
12.2 Oxidation of Alcohols
- PCC will convert a 1 ◦^ alcohol to an aldehyde and oxidize a 2 ◦^ alcohol to a ketone (not useful for 3 ◦)
- KMnO 4 or H 2 CrO 4 (Jones Reagent) can oxidize a 1 ◦^ alcohol to a carboxylic acid
- H 2 CrO 4 can oxidize a 2 ◦^ alcohol to a ketone
12.3 Alcohols from Grignard Reagents
- Grignard Reagents react with any compound that has a hydrogen attached to an atom of high electronegativity (eg: oxygen, nitrogen, sulfur, etc.) and react well with compounds that have carbonyl groups
- Sodium alkynides react with aldehydes and ketones to yield alcohols:
13 Radical Reactions - Mechanisms
13.1 Bromination
- Br 2 and heat/light can perform the following halogen addition to the more substituted carbon of an alkane:
- HBr with a peroxide (ROOR) will have the bromine added to the least substituted carbon of an alkene
13.2 Chlorination
- Cl 2 and heat/light adds a chlorine atom to an alkane; however, it is only useful synthetically when all possible replace- ments yield the same compound (e.g. neopentane and Cl 2 )
