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