Alkyl halides are easily converted to many other functional groups. The halogen atom can leave with its bonding pair of electrons to form a stable __; we say that a halide is a good __.
halide ion
elaving group
When another atom replaces the halide ion, the reaction is a __. When the halide ion leaves another atom or ion and forms a new pi bond, the reaction is an __. In many __, a molecule of H-X is lost from the alkyl halide to give an alkene. These __ are called __ because a hydrogen halide has been removed from the alkyl halide. __ and __ reactions often compete with each other.
substitution
elimination
eliminations
eliminations
dehydrohalogenations
substitution and elimination
In a __, a nucleophile replaces a __ from a carbon atom, using its lone pair of electrons to form a new bond to the carbon atom.
In an __, both the halide ion and another substituent are lost. A new __ is formed.
nucleophilic substitution
leaving group
elimination
pi bond
In the __, the reagent reacts as a __, abstracting a __ from the alkyl halide. Most _ are also basic and can engage in either __ or __, depending on the __ and the reaction conditions.
elimination (a dehydrohalogenation)
base
proton
nucleophiles
substitution or elimination
alkyl halide
Besides __, many other types of compounds undergo __ and __ reactions.
alkyl halides
substitution
elimination
hydroxide ion is a strong __ (donor of an electron pair) because the oxygen atom has unshared pairs of electrons and a negative charge.
__ is called the __, meaning the compound that is attacked by the reagent. The carbon atom of __ is __ because it is bonded to an __.
nucleophile
iodomethane
substrate
iodomethane
electrophilic
electronegative iodine atom
__ is drawn away from carbon by the halogen atom, giving the carbon atom a __. The __ is attracted to this __.
electrond ensity
partial positive charge
negative charge of hydroxide ion
partial positive charge
Hydroxide ion attacks the backside of the __ carbon atom, donationg a pair of electrons to form a new bond.
electrophilic carbon
Carbon can accommodate only eight electrons in its valence shell, so what must happen?
Iodine ion is the __. Why?
the carbon-iodine bond must begin to break as the carbon-oxygen bond begins to form
leaving group
it leaves with the pair of electrons that once bonded it to the carbon atom
What does SN2 stand for?
What does the third term mean?
substitution, nucleophilic, bimolecular
bimolecular means that the transition state of the rate-limiting step involves the collision of two molecules.
Bimolecular reactions usually have __.
rate equations that are second order overall
The __ reaction of methyl iodide with hydroxide ion is a __, taking place in a single step with bonds __ and __ at the same time. THe middle structure is a __, a point of max energy, rather than an __.In this __, the bond to the __ is partially formed, adn the bond to the __ is partially broken.
Sn2 reaction
concerted reaction
breaking and forming
transition state
intermediate
transition state
nucleophile
leaving group
The reaction energy diagram for the substitution sows one?
The reactants are shown slighly __ in energy than the products beacuse this reaction is known to be __. The transition state is much __ in energy because it involves a __ with __.
only one transition state and no intermediates between the reactants and the products
higher
exothermic
higher
five-coordinate carbon atom
two partial bonds
Explain the Sn2 Reaction.
takes place in a single (concerted) step. A strong nucleophile attacks the electrophilic carbon, forcing the leavinggroup to leave
The Sn2 reaction provides a useful method for synthesizing __ and __, which are more difficult to make than __ and __. Halides can be coverted to other halides by __, in which one halide displaces another.
alkyl iodides and fluorides
alkyl chlorides and bromides
halogen exchange reactions
__ is a good nucleophile, and many other alkyl chlorides react with sodium iodide to give alkyl iodides.
iodide
A stronger __ is an ion or molecule that reacts faster in the Sn2 reaction than a "weaker" __ under the same conditions. A strong __ is much more effective than a weak one in attacking an __ carbon atom.
nucleophile x3
electrophile
__ has nonbonding electrons that are readily available for bonding. In the transition state, the negative charge is shared by the oxygen of __ and by hte __.
methoxide ion
methoxide
halide leaving group
__, however, has no negative charge; the transition state has a __ on the halide but a __ on the methanol oxygen aton.
methanol
partial negative charge
partial positive charge
We can generalize the case of methanol and methoxide ion to say what?
a base is always a stronger nucleophile than its conjugate acid
True or False
Nucleophilicity and basicity are synonymous terms.
False
Basicity is defined by the equilibrium constant for abstracting a proton
nucleophilicty is defined by the rate of attack on an electrophilic carbon aton
In both cases, the __ forms a new bond. If the new bond is to a proton, it has reacted as a __; if the new bond is to carbon, it has reacted as __. Predicting which way a species will react may be difficult; most (but not all) good __ are also strong __, and vice versa.
nucleophie (or base)
base
nucleophile
nucleophiles
bases
Trends in nucleophilicity
(1)
a species with a negative charge is a stronger nucleophiel than a similar neutral species. In particular a base is a stronger nucleophile than its conjugate acid
Trends in nucleophilicity
(2)
nucleophilicty decreases from left to right in the periodic table, following the increase in electronegativity from left to right. The more electronegative elements have more tightly held nonbonding electrons that are less reactive toward forming new bonds
Trends in nucleophilicity
(3)
Nucleophilicity incresaes donw the periodic table, following the increase in size andpolarizability, and the decrease in electronegativity
The third trend (__) reflects an atom's ability to engage in partial bonding as it begins to attack an __. As we go down the PT, the atoms become larger, with more electrons at greater distances fromt he nucleus. The electrons are more loosely held, adn the atom is more __: Its electrons can move more freely toward a positive charge, resulting in stronger bonding int he __. The increased mobility of its electrons enhances hte aton's ability to form a bond at a relatively long distance
size and polarizability
electrophilic carbon atom
polarizable
transition state
__ is a "hard" (__) nucleophile, and its nucleus must approach the carbon nucleus quite closely before the elctrons can begin to overlap and form a bond. In the __, there is little bonding between F and C.
fluoride
low-polarizability
transition state
In contrast, the outer shell of the iodide ion is the fifth shell. These electrons are loosely held, making the iodide ion a "soft" __ nucleophie. THe outer electrons begin to shift and overlap with the carbon atom from farther away. There is a great deal of bonding between iodine and carbon int he transition state, which __ the energy of the TS.
high-polarizability
lowers
To serve as a nucleophile, an ion or molecule must do what?
What hinders this, causing a slowing of the reaction rate?
When __ interfere with a reaction by virtue of their size, it is called __.
get in close to a carbon atom to attack it
bulky groups on the nucleophile
bulky groups
steric hindrance
__ has little effect on __ because __ invovles attack on an unhindered proton.
When a __ attacks a carbon atom, however, a bulky __ cannot approach the C atom so easily. Most __ are also __, capable of attacking either a proton or an electrophilic carbon atom.
If we want a species to act as a base, what do we do?
If we want a species to act as a nucleophile, what do we do?
steric hindrance
basicity x2
nucleophile x2
nucleophiles
bases
use a bulky reagent
use a less hindered reagent
Another factor affecting the nucleophilicity of these ions is their __, particularly in __. A __ is one that has acidic protons, usually in the form of O-H or N-H groups. These groups form H bonds to negatively charged __.
__, especially alcohols, are convenient solvents for __ because the reagents tend to be quite __.
solvation
protic solvents x2
nucleophiles
protic solvents
nucleophilic substitutions
soluble
__ are solvated more strongly than __ in a __ becaause the solvent approaches a __ more closely and forms stronger __.
When an anion reacts as a __, energy is __ to __ some of the solvent molecules, breaking some of the hydrogen bonds that stabilized the solvated anion. __ energy is required to __ solvent from a __ such as flouoride than from a __ ion like iodide.
nucleophile
required
strip off
more
strip off
small, strongly solvated
large, diffuse, less strongly solvated
The enahnced solvation of smaller anions in __, requiring more energy to strip off heir solvent molecules, reduces their __..
protic solvents
nucleophilicity
polarizability
This trend reinforces the trend in __: (explain trend)
the polarizability incresaes with increasing atomic number and the solvation energy (in protic solvents) decreases with increasing atomic number. Therefore, nucleophilicity (in protic solvents) generally incresaes down a column in the periodic table, as long as we compare similar species with simar chargnes
In contrast with protic solvents, __ enhance the __ of anions. An anion is more reactive in an __ because it is __.
There are no H bonds to be broken when solvent must make way for the nucleophile to approach an electrophilc carbon atom.
aprotic solvents
nucleophilicity
aprotic solvent
not so strongly solvated
The relatively weak solvating ability of aprotic solvents is also a disadvantage. Why?
most polar, ionic reagents are insoluble in simple aprotic solvents like alkanes
__ have strong dipole moments to enahce solubility, yet they have no O-H or N-H groups to form hydrogen bonds with anions.