Organic Chemistry Alkanes Flashcards
Alkanes are ___ that have the maximum possible number of hydrogen toms attached to each carbon, and thus are said to be saturated. These compounds consist only of hydrogen and carbon atoms joined by single bonds and their general formula is C sub n H sub 2n + 2. However, alkanes can be modified with additional functional groups as well, such as alcohols, halogens, or amines
hydrocarbons
A ___ carbon atom is bonded to only one other carbon atom. A secondary (2 degree) carbon is bonded to two; a tertiary to three, and a quaternary to four other carbon atoms. In addition, hydrogen atoms and other functional groups attached to 1 degree, 2 degree, or 3 degree carbon atoms are referred to as 1 degree, 2 degree, or 3 degree, respectively
primary
In general, as the ___ of a straight chain alkane increases, the melting point, boiling point, and density also increase. Branched molecules have slightly lower boiling and melting points than their straight-chain isomers. Greater branching reduces the surface area of a molecule, decreasing the weak intermolecular attractive forces (London dispersion forces). The molecules are held together less tightly, thus lowering the boiling point. In addition, branched molecules are more difficult to pack into a tight, three-dimensional structure, which is reflected in the lower melting points of branched alkanes
molecular weight
___ and other substituted carbon molecules can take part in reactions known as nucleophilic substitutions. Substitution reactions involve removing an atom or a functional group from a molecule and replacing it with another.
alkyl halides
___ are molecules that are attracted to positive charge, as seen by their name: nucleophile means “nucleus lover.” Nucleophiles are electron-rich species that are often but not always negatively charged. Nucleophiles are attracted to atoms with partial or full positive charges
nucleophiles
If a group of nucleophiles are based on the same atom then ___ is roughly correlated to basicity. In other words, the stronger the base, the stronger the nucleophile. This is because bases act as electron donors, and stronger nucleophiles are also better electron donors. Nucleophilic strength decreases in the order:
RO- > HO- > RCO2 - > ROH > H20
nucleophilicity
If a series of ___ is based on different atoms, nucleophilic ability doesn’t necessarily correlate to basicity. In a protic solvent (a solvent that 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. Hence, nucleophilic strength decreases in the order:
CN- > I- > RO- > HO- > Br- > Cl- > F- > H2O
In contrast, in an aprotic solvent (a solvent that cannot form hydrogen bonds) the nucleophiles are not solvated. In this case, nucleophilic strength is directly related to basicity. In DMSO (an aprotic solvent), the order of nucleophilic strength is the same as base strength:
F- > Cl- > Br- > I-
nucleophiles
The ease with which ___ substitution takes place is dependent on the nature of the leaving group. The best leaving groups are those that are weak bases, as these accept a negative charge and dissociate to form a stable ion in solution. In the case of the halogens, therefore, this is the opposite of base strength:
I- > Br- >Cl- > F-
An OH group that is a poor leaving group can be protonated to form H2O+, which leaves as water molecule
nucleophilic
SN1 designates a ___ 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 of an SN1 reaction is the dissociation of the substrate (the starting molecule) to form a stable, positively charged ion called a carbocation. The formation and stabilization of the carbocation determine all other aspects of SN1 reactions
unimolecular nucleophilic substitution
SN1 reactions involve two steps: the dissociation of a ___ molecule into a carbocation and a leaving group, followed by the combination of the carbocation with a nucleophile to form the substituted product
substrate
In the first step, a carbocation intermediate is formed. ___ are stabilized by polar solvents that have lone electron pairs available to donate (e.g., water or ethyl alcohol). Carbocations are also stabilized by charge delocalization throughout the molecule. More highly substituted carbocations are more stable, because hydrocarbon substituent groups donate electron density toward the positive charge. The order of stability for carbocations is:
tertiary > secondary > primary > methyl
to drive the reaction forward, the original leaving group should be a weaker nucleophile than the replacement nucleophile. The second step, in which the nucleophile combines with the carbocation, occurs very rapidly compared to the first step, and is essentially irreversible
carbocations
The ___ at which a reaction occurs can never be greater than the rate of its slowest step. Such a step is termed the rate-limiting or rate-determining step of the reaction. In an SN1 reaction, the slowest step is the dissociation of the molecule to form a carbocation intermediate, a step that is energetically unfavorable. The formation of a carbocation is therefore the rate-limiting step of an SN1 reaction. The only reactant in this step is the original substrate molecule, and so the rate of the entire reaction depends only on the concentration of the substrate (a so-called first order reaction): rate = k[substrate]. The rate is not dependent on the concentration or the nature of the nucleophile, because it plays no part in the rate-limiting step
rate
The rate of an SN1 reaction can be increased by anything that promotes the formation and stability of the ___. The most important factors are as follows:
A. Structural factors: highly substituted alkanes allow for distribution of the positive charge over a greater number of carbon and hydrogen atoms, and thus form the most stable carbocations. The order of reactivity of substrates for SN1 reactions is tertiary > secondary > primary > methyl; in general, primary and methyl substrates do not react by the SN1 mechanism
B. Solvent effects: highly polar solvents are better a surrounding and isolating ions than are less polar solvents. Polar protic solvents such as water or alcohols work best for two reasons. Protic solvents can form hydrogen bonds with the leaving group, solvating it and preventing it from returning to the carbocation. Also, lone electron pairs on oxygen or nitrogen atoms in the solvent molecule can stabilize the carbocation intermediate
C. Nature of the leaving group: weak bases dissociate more easily from the alkyl chain and thus make better leaving groups, increasing the rate of carbocation formation
D. Nature of the nucleophile: 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
carbocation
SN1 reactions involve carbocation ___, which are sp2 hybridized and have trigonal planar geometry. The attacking nucleophile can approach the carbocation from either above or below with equal probability, and thus create either the (R) or (S) enantiomer with equal probability.
If the original compound is optically active because of a chiral center, then a racemic mixture will be produced. In some cases, the nucleophile may react so rapidly that it approaches the carbocation from the opposite side of the leaving group with greater frequency; in this case, the product will be characterized as a partial racemate
intermediates
SN2 designates a bimolecular ___ substitution reaction. SN2 reactions involve a nucleophile pushing its way into a compound while simultaneously displacing the leaving group. Its rate-determining and only step involves two molecules: the substrate and the nucleophile
nucleophilic
