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OChem Ch.7
| Question | Answer |
|---|---|
| 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? | ? |
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