What is the difference between an addition, substitution, and elimination reaction? Solution: Addition reactions involve the combination of two or more molecules to form a larger one. Substitution reactions involve the replacement of an atom or group of atoms in a molecule by another atom or group of atoms. Elimination reactions involve the removal of atoms or groups from a molecule, often resulting in the formation of a double or triple bond.
- What is a nucleophilic substitution reaction? Solution: A nucleophilic substitution reaction is a type of chemical reaction where a nucleophile, a chemical species rich in electrons, replaces a leaving group in a molecule.
- How does a molecule’s structure affect whether it undergoes an addition, substitution, or elimination reaction? Solution: Molecules with multiple bonds, like alkenes and alkynes, typically undergo addition reactions. Molecules like alkyl halides or alcohols can undergo nucleophilic substitution or elimination reactions based on conditions and the nature of the attacking species.
- What is Markovnikov’s rule and how does it apply to addition reactions? Solution: Markovnikov’s rule states that in the addition of a protic acid HX to an alkene, the acid hydrogen (H) becomes attached to the carbon with the fewest alkyl substituents, and the halide (X) group goes to the carbon with the most alkyl substituents. It is used to predict the outcome of addition reactions.
- What is an SN2 reaction and what factors favor it? Solution: An SN2 reaction is a type of nucleophilic substitution reaction where the rate determining step involves two chemical species. Factors that favor it include the use of a strong nucleophile, a good leaving group, a polar aprotic solvent, and a primary or secondary carbon.
- How does an E1 reaction differ from an E2 reaction? Solution: An E1 reaction is a type of elimination reaction where the removal of the leaving group is the rate-determining step, forming a carbocation intermediate. An E2 reaction is a type of elimination reaction where the proton transfer and the leaving group removal occur in a single concerted step.
- How does an SN1 reaction differ from an SN2 reaction? Solution: An SN1 reaction involves two steps – formation of a carbocation intermediate by departure of the leaving group, then attack by the nucleophile. It is usually observed with tertiary carbon atoms. An SN2 reaction is a single-step reaction where the nucleophile attacks the carbon at the same time as the leaving group departs. It is usually observed with primary and secondary carbon atoms.
- What is a leaving group and what makes a good leaving group in substitution and elimination reactions? Solution: A leaving group is an atom or group of atoms that is displaced as a stable species during a substitution or elimination reaction. Good leaving groups are usually weak bases that can stabilize the negative charge, such as halides or water.
- What is the role of the solvent in substitution and elimination reactions? Solution: The solvent can play a significant role in determining the mechanism (SN1 vs SN2 or E1 vs E2) and the rate of the reaction. Polar protic solvents (like water or alcohols) can stabilize carbocation intermediates and favor SN1 and E1 reactions. Polar aprotic solvents (like DMSO or acetone) do not stabilize carbocations but can solvate the nucleophile, favoring SN2 and E2 reactions.
- How can you predict whether a substitution or elimination reaction will occur? Solution: The likelihood of substitution versus elimination depends on several factors: the structure of the substrate (primary, secondary, or tertiary carbon), the strength and bulkiness of the base/nucleophile, the solvent, and the temperature. For instance, bulky bases and high temperatures favor elimination.
- What is a concerted reaction and how does it apply to substitution and elimination reactions? Solution: A concerted reaction is a reaction where all bond breaking and bond forming occurs in a single step, with no intermediate species formed. SN2 and E2 reactions are examples of concerted reactions.
- How does the use of a strong base affect whether a substitution or elimination reaction occurs? Solution: The use of a strong base typically favors elimination reactions, as the base can abstract a proton from the β-carbon of the substrate, leading to formation of a double bond.
- What is the difference between a regioselective and a stereoselective reaction? Solution: A regioselective reaction is one in which one direction of bond forming or breaking is preferred over all others, while a stereoselective reaction is one in which one stereoisomer is formed in preference to all others. Both types of selectivity can play a role in addition, substitution, and elimination reactions.
- What is the Zaitsev’s rule in elimination reactions? Solution: Zaitsev’s rule states that in an elimination reaction, the most substituted product will be the most stable, and therefore the most likely to form.
- What are the products of the addition of bromine to propene? Solution: The product of the addition of bromine (Br2) to propene (CH3CH=CH2) is 1,2-dibromopropane (CH3CHBrCH2Br), following Markovnikov’s rule.
- Why do alkenes undergo addition reactions more readily than alkanes? Solution: Alkenes have a region of high electron density in the pi bond between the carbon atoms, which makes them more reactive towards electrophiles. Alkanes, on the other hand, have only single bonds and are relatively inert.
- What happens when methane reacts with chlorine in the presence of light? Solution: Methane reacts with chlorine in the presence of light (a source of energy) to undergo a substitution reaction, replacing a hydrogen atom with a chlorine atom to form chloromethane.
- What happens in the reaction of 2-bromopropane with potassium hydroxide? Solution: 2-bromopropane reacts with potassium hydroxide to undergo an E2 elimination reaction, removing the bromine atom and a hydrogen atom from the adjacent carbon atom to form propene.
- What happens in the reaction of bromoethane with sodium hydroxide? Solution: Bromoethane reacts with sodium hydroxide to undergo an SN2 substitution reaction, replacing the bromine atom with an OH group to form ethanol.
What is the major product of the reaction of 2-methylpropene with water in the presence of an acid catalyst? Solution: The major product of the reaction of 2-methylpropene (CH3C(CH3)=CH2) with water in the presence of an acid catalyst is 2-methylpropan-2-ol (CH3C(CH3)(OH)CH3), following Markovnikov’s rule.