40 Questions
What is the result of reacting tertiary alcohols with HX?
Fast and effective reaction
Why are alternative methods used for primary and secondary alcohols?
They react very slowly and often rearrange
What is used to prepare alkyl halides from primary and secondary alcohols?
SOCl2 or PBr3
What type of reaction do alkyl halides undergo with nucleophiles?
Substitution
What type of reaction do alkyl halides undergo with Brønsted-Lowry bases?
Elimination
How many components are necessary in any substitution reaction?
Three
What is the purpose of adding HCl or HBr gas into an ether solution of tertiary alcohol?
To prepare alkyl halides
What is formed when a tertiary alcohol reacts with HX?
Alkyl halide
Why are phosphorous halides used in the preparation of alkyl halides?
They are useful for primary and secondary alcohols
What is the general term for the reaction of an alkyl halide with a nucleophile or a base?
Substitution or elimination
What is the kinetic order of an SN2 reaction?
Second order
What is the effect of bulky R groups on the SN2 reaction rate?
Decrease the reaction rate
Why do 3° alkyl halides not undergo SN2 reactions?
Due to steric effects
What is the key feature of the SN1 mechanism?
Two-step mechanism
What is the role of the nucleophile in the SN1 reaction rate?
Has no effect on the reaction rate
What is the type of reaction mechanism exemplified by reaction 1?
SN2
What is the effect of the number of R groups on the carbon with the leaving group on the SN2 reaction rate?
Decreases the reaction rate
What is the reactivity of methyl and 1° alkyl halides in SN2 reactions?
They react with ease
What is the kinetic order of an SN1 reaction?
First order
What is the characteristic of the reaction site in an SN2 reaction?
Attack from the back
What is the most common type of base used in elimination reactions?
Negatively charged oxygen compounds
How do you identify the α carbon in an elimination reaction?
By finding the carbon with the leaving group
What is the result of removing the elements of H and X from the α and β carbons in an elimination reaction?
Formation of a π bond
How many mechanisms of elimination are there?
Two
What is the name of the bimolecular elimination mechanism?
E2
What is the name of the unimolecular elimination mechanism?
E1
What is the relationship between the E2 and SN2 mechanisms?
They have some features in common
What is the relationship between the E1 and SN1 mechanisms?
They have some features in common
What is the first step in drawing the product of dehydrohalogenation?
Identify the α carbon
What is the purpose of identifying the β carbon in an elimination reaction?
To identify the site of H atom removal
I¯ is a good nucleophile and a strong base.
False
DBU is a bulky nucleophilic base that favors substitution.
False
CH3COO¯ is a strong base that favors elimination.
False
KOC(CH3)3 is a good nucleophile that favors substitution.
False
Tertiary alcohols react with HX to form alkyl halides.
True
Bulky R groups increase the SN2 reaction rate.
False
E2 reactions involve the formation of a carbocation intermediate.
False
HS¯ is a weak base that favors substitution.
True
DBN is a good nucleophile that favors substitution.
False
Br¯ is a strong base that favors elimination.
False
Study Notes
Alkyl Halides
- Alkyl halides are organic molecules containing a halogen atom bonded to an sp3 hybridized carbon atom.
- The halogen atom is often denoted by the symbol X.
- Alkyl halides are classified as primary (1°), secondary (2°), or tertiary (3°), depending on the number of carbons bonded to the carbon with the halogen atom.
Types of Alkyl Halides
- Vinyl halides have a halogen atom bonded to a C-C double bond.
- Aryl halides have a halogen atom bonded to a benzene ring.
- Allylic halides have X bonded to the carbon atom adjacent to a C-C double bond.
- Benzylic halides have X bonded to the carbon atom adjacent to a benzene ring.
Nomenclature of Alkyl Halides
- Common names are often used for simple alkyl halides.
- To assign a common name:
- Name all the carbon atoms of the molecule as a single alkyl group.
- Name the halogen bonded to the alkyl group.
- Combine the names of the alkyl group and halide, separating the words with a space.
Polarity of Alkyl Halides
- Alkyl halides are weakly polar molecules.
- They exhibit dipole-dipole interactions due to their polar C-X bond.
- They are incapable of intermolecular hydrogen bonding.
Simple Alkyl Halide of Medical Importance
- Halothane (CF3CHCLBr) is a safe general anesthetic that has replaced other organic anesthetics.
The Polar Carbon-Halogen Bond
- The electronegative halogen atom in alkyl halides creates a polar C-X bond, making the carbon atom electron deficient.
- Electrostatic potential maps illustrate this point.
Preparing Alkyl Halides
- From alkanes: Radical halogenation replaces C-H with C-X, but gives mixtures.
- From alkenes: Addition of HX to alkenes gives Markovnikov product.
- From alcohols: Reaction of tertiary C-OH with HX is fast and effective.
Reaction Types for Alkyl Halides
- Alkyl halides undergo substitution reactions with nucleophiles.
- Alkyl halides undergo elimination reactions with Brønsted-Lowry bases.
Substitution Reactions
- Three components are necessary: sp3 hybridized carbon with the leaving group, nucleophile, and leaving group.
- To draw any nucleophilic substitution product: find the sp3 hybridized carbon with the leaving group, identify the nucleophile, and substitute the nucleophile for the leaving group.
Nucleophiles
- Nucleophiles are Lewis bases that can be negatively charged or neutral.
- Examples: HO-, HS-, and CH3COO-.
- Nucleophiles differ from bases in what they attack.
Leaving Groups
- The weaker the base, the better the leaving group.
- Good leaving groups: H2O, Cl-, Br-, I-, and ROSO3-.
- Poor leaving groups: HO-, RO-, and H-.
SN2 Mechanism
- Concerted mechanism with a one-step process.
- Rate depends on the concentration of both reactants.
- Rate equation is second order.
SN1 Mechanism
- Two-step mechanism with a carbocation intermediate.
- Rate depends on the concentration of RX only.
- Rate equation is first order.
Kinetics and Mechanisms
- Consider reaction kinetics to determine the mechanism.
- SN2 reactions exhibit second-order kinetics, while SN1 reactions exhibit first-order kinetics.
Steric Effects on Nucleophile Strength
- Steric hindrance decreases nucleophilicity but not basicity.
- Sterically hindered bases that are poor nucleophiles are called nonnucleophilic bases.
Bond Breaking and Making in Nucleophilic Substitution Mechanisms
- There are three possibilities: bond making and breaking occur at the same time, bond breaking occurs first, or bond making occurs first.
Predicting the Mechanism of Nucleophilic Substitutions
- Four factors are relevant in predicting the mechanism: the alkyl halide, the nucleophile, the leaving group, and the solvent.
SN1 and E1 Reactions
- Both have the same first step: formation of a carbocation.
- They differ in what happens to the carbocation.
Learn about alkyl halides, their classification, and types. Understand the structure and properties of primary, secondary, and tertiary alkyl halides, and vinyl halides. This quiz covers the basics of alkyl halides in organic chemistry.
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