Quiz #4 Flashcards
How do the boiling points of alkyl halides where X = I, Br, or CL relate to their corresponding alkanes?
Their boiling points are higher due to their increased dipole intermolecular forces
Are the densities of alkyl halides where X = Br or I greater than or less than water?
Greater than water
Are the densities of alkyl halides with 2+ Cl greater than or less than water?
Greater than water
How do the C-X bonds of alkyl halides relate to their corresponding C-H bonds
For all X EXCEPT F:
The C-X bonds are weaker, longer, and easier to break
Does free radical halogenation of alkanes have regioselectivity?
YES!
Halogenation of 3’ is preferred over 2’ is preferred over 1’ substituted C
Does free radical halogenation of alkanes have sterioselectivity?
NO!!!
Free radicals are NOT stereoselective! They can react from above or from below!
Definition of a free radical
A chemical species with 1 or more unpaired electrons
Are free radicals stereoselective?
NO!!! They can react from either above or below
Steps of radical rxn mechanism
1) Initiation
2) Propagation
3) Termination
Why are NCS and NBS used as Cl* and Br* sources?
Using them allows you to avoid having to use high temps to form the initial radical and initiate the radical reaction
What is the major product in radical allylic halogenation rxns?
Major product is the most substituted, and therefore must stable, alkene
At what speed do radical reactions happen?
Very quickly
Rank the following radicals in order of increasing stability:
1’ – 2’ – 3’
Least stable
1’
2’
3’
Most stable
Are more substituted radicals more or less stable?
More stable. Substitution increases radical stability
When under the right conditions, where can alkenes be halogenated (via radicals)?
At their allylic positions, carbons adjacent to the double bond
Why do allylic H’s react with radicals so easily?
Because the allyl radical formed in the rxn exists in a p-orbital stabilized by resonance with the alkene double bond
Definition of nucleophilic substitution reactions
A nucleophile replaces the LG on an sp3 C of an electrophile
Leaving Group (LG)
Any group that can be expelled
Functions of the LG
1) It withdraws electron density from the C to which it is attached, rendering the C electrophilic
2) It stabilizes the negative charge after being expelled
What does nucleophilicity measure?
Kinetics, how fast something happens
What does the tallest hump of an energy diagram represent?
The rate-determining step
What does the 1 in Sn1 represent?
Means that the rate of an Sn1 rxn is dependent only on 1 variable (concentration of electrophile/LG)
What does the 2 in Sn2 represent?
Means that the rate of an Sn2 rxn is dependent on 2 variables (concentrations of both electrophile/LG AND nucleophile)
How are carbocations stabilized?
Substitutions (3’, 2’, 1’)
Resonance (benzylic positions, allylic positions)
