PCAT: Chapter 39 - Alkanes
In an ___ solvent (a solvent that cannot form hydrogen bonds) the nucleophiles are "naked", they are not solvated. In this case, nucleophilic strength is directly related to basicity.
Aprotic
___ designates a unimolecular nucleophilic substitution reaction. It is unimolecular because the rate of the reaction is dependent upon only one molecule in the reaction; in other words, the rate expression is first order. The rate-determining step is the dissociation of the substrate (the starting molecule) to form a stable, positively charged ion called a ___. The formation and stabilization of the carbocation determine all of the other aspects of this reaction.
Sn1; carbocation
___ designates a bimolecular nucleophilic substitution reactions. These involve a nucleophile pushing its way into a compound while simultaneously displacing the leaving group.
Sn2
hydrocarbons that have the maximum possible number of hydrogen atoms attached to each carbon, and thus are said to be saturated. These compounds consist only of hydrogen and carbon atoms joined by single bonds.
alkanes
A tertiary radical is the most likely to be formed in a free-radical ___. As a result, the substitution product will have the bromine on the most highly substituted carbon.
bromination
free-radical ___ is a more rapid process and depends on both the stability of the intermediate and on the number of hydrogens present. These reacations are likley to replace primary hydrogens that are found abundantly in most molecules, despite the relative instability of primary radicals.
chlorination
The equation for the complete ____ of propane is: C₃H₈ + 5 O₂ --> 3 CO₂ + 4 H₂O + heat
combustion
Sn2 reactions are ___ reactions, meaning that the entire mechanism occurs in a single coordinated process. The nucleophile attacks the reactant from the backside of the leaving group, forming a trigonal bipyramidal ___ ___.
concerted; transition state
At room temperature, the straight-chain compounds C₁H₄ through C₄H₁₀ are ___, C₅H₁₂ through C₁₆H₃₄ are ___ and the longer-chain compounds are waxes and harder ___.
gases; liquids; solids
As the molecular weight of a straight chain alkane increases, the melting point, boiling point, and density ____.
increase
___ step of free radical substitution: Diatomic halogens are cleaved by either UV light (shown as UV or hv) or peroxide (HOOH or ROOH), resulting in the formation of free radicals (heat can also be used, but is not specific for radical formation). Free radicals are uncharged species with unpaired electrons (such as Cl∙, or R₂C∙). They are extremely reactive and readily attack alkanes. X₂ -(hv)-> 2X∙
initiation
Nature of the ___ ___ in Sn1 reactions: Weak bases dissociate more easily from the alkyl chain and thus make better leaving groups, increasing the rate of carbocation formation.
leaving group
Nature of the ___ in Sn2 reactions: Weak bases dissociate more easily from the alkyl chain and thus make better leaving groups, increasing the ease of displacement by the nucleophile.
leaving group
The ease with which nucleophilic substitution takes place is dependent on the nature of the ___ ___. The best ones are those that are weak bases, as these can accept a negative charge and dissociate to form a stable ion in solution.
leaving groups
Branched molecules have slightly ___ boiling and melting points than their straight-chain isomers.
lower
Nature of the ___ in Sn2 reactions: Because the nucleophile must attack a neutral molecule and displace the leaving group, it must be strongly nucleophilic. This typically means that they nucleophile is a negatively charged ionic species, and can be either a strong or weak base.
nucleophile
Nature of the ___ in Sn1 reactions: Sn1 reactions do not require a strong nucleophile. Sn1 reactions run equally well with either strong (fully charged) or weak (electron-rich but uncharged) nucleophiles.
nucleophiles
___ are molecules that are attracted to positive charge, as seen by their name. They are electron-rich species that are often but not always negatively charged. They are attracted to atoms with partial or full positive charges.
nucleophiles
Alkhyl halides are other substituted carbon molecules can take part in reactions known as ___ ___.
nucleophilic substitutions
A ____ carbon (1°) is bonded to only one other carbon atom.
primary
___ step of free radical substitution: One in which a radical produces another radical that can continue the reaction. A free radical reacts with an alkane, removing a hydrogen atom to form HX, and creating an alkyl radical. The alkyl radical can react with X₂ to form an alkyl halide (the substituted produce) and generate another X∙. X∙ + RH --> HX + R∙ R∙ + X₂ --> RX + X∙
propagation
In a ___ solvent (a solvent is able to form hydrogen bonds), large atoms or ions tend to be better nucleophiles. Larger ions more easily shed their solvent molecules and are more polarizable.
protic
___ occurs when a molecule is broken down by heat in the absence of oxygen.
pyrolysis
a ____ carbon (4°) is bonded to four other carbon atoms
quaternary
The rate at which a reaction occurs can never be greater than the rate of its slowest step. Such a step is termed the ___-___ or ___-___ step of the reaction. In an Sn1 reaction, this step is the formation of a carbocation.
rate-limiting or rate-determining
Since the rate of the Sn2 reaction depends on the concentration of two reactants, it follows ___-order kinetics. rate = k[substrate][nucleophile]
second
A ____ carbon (2°) is bonded to two other carbons
secondary
If a group of nucleophiles are based on the same atom (e.g. oxygen) then nucleophilicity is roughly to basicity. In other words, the ___ the base, the stronger the nucleophile.
stronger
___ factors of an Sn1 reaction: Highly substituted alkanes allow for distribution of the positive charge over a greater number of carbons and hydrogen atoms, and thus form the most stable carbocations.
structural
___ reactions involve removing an atom or a functional group from a molecule and replacing it with another. They can occur in many different types of molecules.
substitution
___ step of free radical substitution: Two free radicals combine with one another to form a stable molecule. X∙ + X∙ --> X₂ X∙ + R∙ --> RX R∙ + R∙ --> R₂
termination
A ____ carbon (3°) is bonded to three other carbons
tertiary
Alkanes can react by a free radical substitution mechanism in which one or more hydrogen atoms are replaced by Cl, Br, or I atoms. These reactions involve three steps:
Initiation, propagation, termination
In a process called ___, a radical transfers a hydrogen atom to another radical, producing an alkane and an alkene. ∙CH₃ + ∙CH₂CH₃ --> CH₄ + H₂C=CH₂
disproportionation