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OChem Ch.7

QuestionAnswer
Another name for a haloalkane Alkyl Halide
What's a haloalkane? a compound containinga halogen atom covalently bonded to an sp3 hybridized carbon
Why are haloalkanes polar compounds? because of the higher electronegativity of the halogens (except for iodine) compared to carbon
In a haloalkane, which has the partial positive charge and which has the partial negative charge between the carbon and the halogen? (there is an exception) Carbon carries the partial positive charge and the halogen carries the partial negative charge
What's the structure of a freon? Chlorofluorocarbon (CFCs). Example: CCl3F
What are freons used as and why is it badthat they're chemically inert? They're used as refrigerants, solvents, flame retardents, etc. And because they're chemically inert, they don't decompose in the lower atmosphere, so they rise to the stratosphere.
What's an alkene? It contains only carbons and hydrogens, and it has one carbon-carbon double bond
What are the two types of reactions of haloalkanes? Nucleophilic substitution reactions & B-elimination reactions
What's a nucleophile? A reagent that forms a chemical bond with its partner by donating both bonding electrons.
What does the general reaction look like of a nucleophilic substitution reaction? Nu:- + CH3X --> CH3NU + :X-
What's the equation for a reaction rate in an Sn2 nucleophilic substitution reaction? Reaction rate = k[haloalkane][nucleophile]
What does the rate of reaction of an Sn2 nucleophilic substitution reaction depend on? The concentration of the haloalkane AND the nucleophile
What is a fundamental difference between the two mechanisms for nucleophilic substitutions? the timing of the bond-breaking and bond-forming steps
In an Sn2 reaction, are one or both reactants involved in the transition state of the rate-determining step? Both reactants
What does an Sn2 reaction on a stereocenter result in? A *Walden Inversion (inversion of the configuration)
Why is the reaction kinetic of an Sn2 reaction bimolecular? Both the nucleophile and the leaving group are part of the slowest step in the reaction
What's an Sn1 reaction? This means that the bond breaking between the carbon and the leaving group is entirely completed before the bond forming with the nucleophile begins
What's the equation for a reaction rate in an Sn1 nucleophilic substitution reaction? Reaction rate = k * [haloalkane]
What does the rate of reaction of an Sn1 nucleophilic substitution reaction depend on? The concentration of the haloalkane ONLY
What's an electrophile? A reagent involved in a chemical reaction that accepts an electron pair in order to bond to a nucleophile. It's "attracted" to electrons
What's the first step of an Sn1 reaction? Ionization of the C-X bond gives a carbocation intermediate (the electrophile); a + (on the carbocation intermediate) and - (halogen ion) charge is formed because of ionization
What's the second step of an Sn1 reaction? Reaction of the carbocation (electrophile) with a nucleophile; The positive charge [on the carbon] is transferred to the nucleophile
What's the third step of an Sn1 reaction? Proton transfer completes the reaction (and neutrality is once again formed)
What is the reaction rate determining step of an Sn1 reaction? the Carbocation formationcarbocation formation is the reaction rate determining step
Why is the carbocation formation the reaction rate determining step of an Sn1 reaction? E_a1 > E_a2
Nucleophilicity A kinetic property that measures the rate at which a nucleophile attacks a reference compound under a standard set of experimental conditions
Relationship between reaction rate and the nucleophilicity of the nucleophile The higher the reaction rate, the bigger the nucleophilicity of the nucleophile
What are two different properties that can make a nucleophile more nucleophilic? 1) they're an anion instead of neutral 2) They have a higher basicity (affinity to protons)
In which mechanism does the nucleophile participate in the rate-determining step? *[why?] Sn2 mechanism [?]
In which mechanism is the reaction rate independent of the nucleophilicity of the nucleophile? Sn1 mechanism
Do stronger nucleophiles Sn1 or Sn2 mechanisms? Sn2 mechanisms
A reaction will be more likely to proceed according to an Sn1 mechanism if the carbocation intermediate stability is --> ? higher or lower? higher?
What is the order of the stability and rates of the carbocations in an Sn1 reaction? tertiary haloalkane > secondary haloalkane > primary haloalkane > methylhaloalkane
Sn1 reactions are governed by ___ factors and Sn2 reactions are governed by ___ factors. electronic; steric
What are "electronic factors" (Sn1 reaction)? The relative stabilities of the carbocation intermediates
What are "steric factors" (Sn2 reaction)? The relative ease of approach of the nucleophile to the site of reaction
What is the order of the reactivity of Sn2 reactions? methyl > primary > secondary > tertiary
What is the leaving group? It's the part of the molecule that is substituted by the nucleophile.
Are the conjugates of strong acids, strong bases, weak acids, or weak bases usually good leaving groups? The conjugates of strong acids (which are weak bases)
Put in order the relative leaving abilities of these Halide ions: Br-, Cl-, F-, I- I- > Br- > Cl- > F-
Are the conjugates of strong acids, strong bases, weak acids, or weak bases usually poor leaving groups? The conjugates of weak acids (which are strong bases)
What are 3 examples of poor leaving groups, and what do they have in common? OH-, RO-, and NH2-; They're all strong bases
What's a protic solvent? A solvent that contains an -OH or an -NH group
What do protic solvents do to carbocations? They stabilize the carbocations through solvation (dipole-dipole interactions)
What's an aprotic solvent? It doesn't contain an -OH or an -NH group
Do aprotic solvents favor Sn1 or Sn2 reactions? Sn2 reactions?
With what type of haloalkanes are Sn2 reactions favored? methyl and primary
With what type of haloalkanes are Sn1 reactions favored? secondary and tertiary
In an Sn2 reaction, what does the substitution at a stereocenter cause? Inversion of configuration
In an Sn1 reaction, what does the substitution at a stereocenter cause? Racemization
What is "inversion of configuration"? When the nucleophile attacks the stereocenter from the side opposite from the leaving group
What is "racemization"? When the carbocation intermediate is planar, and the attack by the nucleophile occurs with equal probability from either side
What's a Beta-elimination reaction? The removal of atoms or groups of atoms from adjacent carbons to form a carbon-carbon double bond
What's another name for dehydrohalogenation, and what is it? An organic reaction in which an alkene is obtained from an alkyl halide
What's an alkyl halide? An organic compound in which halogen atoms have been substituted for hydrogen atoms in an alkane
*Zaitsev rule If several B-hydrogen atoms are available for elimination, the major product is the more stable (the more highly substituted) alkene
What are the two limiting mechanisms for B-elimination reactions? E1 and #2
What types of bases are used in B-elimination? Give examples Strong bases, like OH-, OR-, and NH2-
What is an E2 reaction? The breaking of the C-X and C-H bonds is concerted (simultaneous)
What is involved in the rate-determining step of an E2 mechanism? R-X and the base
What does the reaction rate of an E2 mechanism depend on? the concentration of the haloalkane AND the base
What's the equation for the reaction rate of an E2 reaction? reaction rate = k[haloalkane][base]
How many steps are in an E2 mechanism, and why? 1; all bond-breaking and bond-forming steps are concerted
What's an E1 mechanism? The breaking of the C-X bond is complete before the reaction with the base breaks the C-H bond
What's involved in the rate-determining step of an E1 mechanism reaction? R-X
What's the equation for the reaction rate of an E1 reaction? Reaction rate = k*[haloalkane]
What's the first step of an E1 mechanism? The ionization of C-X gives a carbocation intermediate (with a positively-charged C and a negatively-charged halogen ion (halide ion?))
What's the second & final step of an E1 mechanism? There is a proton transfer from the carbocation intermediate to the base, and the alkene is formed
*Why do 3* haloalkanes undergo E2 reactions with strong bases whereas they never react according to the SN2 mechanism? ?
Created by: 817229501