MCAT ORGO II Flashcards

Terms Definitions
are alkanes that contain one or more members of the halogen family. The halogens found in organic molecules are chlorine, bromine, fluorine, and iodine. Some texts refer to this class of compounds as halogenoalkanes or alkyl halides.
C—F > C—Cl > C—Br > C—I
R-X bond polarity:
Haloalkanes are generally liquids at room temperature. Haloalkanes generally have a boiling point that is _______ than the alkane they are derived from. This is due to the increased molecular weight due to the large halogen atoms and the increased intermo
C—F < C—Cl < C—Br < C—I
R-X bond length
C—F > C—Cl > C—Br > C—I (not polarity)
Bond strength
The orbitals C uses to make bonds are
2s and 2p. The overlap integral is larger the closer the principal quantum number of the orbitals is, so the overlap is larger in the bonds to lighter halogens, making the bond formation energetically favorable.
C—F < C—Cl < C—Br < C—I
Bond reactivity
A famous test used to determine if a compound is a haloalkane is the ___________, in which the compound tested is burned in a loop of copper wire. The compound will burn green if it is a haloalkane. The numbers of fluorine, chlorine, bromine and iodine at
Beilstein test
R-X bonds are very commonly used throughout organic chemistry because their polar bonds make them reasonably reactive. In a ___________, the halogen (X) is replaced by another substituent (Y). The alkyl group (R) is not changed.
substitution reaction
Y: + R—X → R—Y + X:
A general substitution reaction
Substitutions involving haloalkanes involve a type of substition called__________ __________, in which the substituent Y is a ________ . A _________is an electron pair donor. The _________replaces the halogen, an electrophile, which becomes a leaving grou
Nucleophilic substitution, nucleophile
Nu represents a generic nucleophile.
Nu:- + R—X → R—Nu + X:-
Nu: + R—X → R—Nu+ + X:-
General nucleophilic substitution reactions
:O-H Hydroxide R—OH Alcohol
:O-R Alkoxide R—O—R Ether
:S-H Hydrosulfide R—SH Thiol
:NH3 Ammonia R—NH3+ Alkylammonium ion
:C-N Cyanide R—CN Nitrile
:C≡C—H Acetylide R-C≡C—H Alkyne
:I- Iodide R—I AlkylIodide
:R- Carbanion
Common Nucleophiles
Nucleophilic substitution can occur in two different ways. SN2 involves a _______ ________. SN1 involves a ________ _________.
backside attack, carbocation intermediate
Rate depends on concentrations of both the haloalkane and the nucleophile
SN2 reaction
Rate depends only on the concentration of the haloalkane. The carbocation forms much slower than it reacts with other molecules.
SN1 reaction
Configuration is inverted (i.e. R to S and vice-versa).
SN2 stereochemistry
Polar aprotic solvents favored. Examples: Acetone, THF (an ether), dimethyl sulfoxide, n,n-dimethylformamide, hexamethylphosphoramide (HMPA). Nonpolar solvents will also work, such as carbon tetrachloride (CCl4)
Polar protic solvents favored. Examples: H2O, Formic acid, methanol. Aprotic solvents will work also, but protic solvents are better because they will stabilize the leaving group, which is usually negatively charged, by solvating it. Nonpolar solvents are
Role of solvent:
Good nucleophiles favored
Role of nucleophile: SN2
Any nucleophile will work (since it has no effect on reaction rate)
Role of nucleophile: SN1
3° carbon - most stable
= SN1 favored
2° carbon less stable =
either could be favored
1° carbon - seldom forms =
SN2 favored
The reason why the tertiary carbocation is most favored is due to
the inductive effect. In the carbocation intermediate, there is a resulting formal charge of +1 on the carbon that possessed the haloalkane. The positive charge will attract the electrons available. Because this is tertiary, meaning that adjacent carbon atoms and substituents are available, it will provide the most electron-density to stabilize this charge.
created by reacting magnesium metal with a haloalkane. The magnesium atom gets between the alkyl group and the halogen atom with the general reaction:
R-Br + Mg → R-Mg-Br
Grignard reagents
With alcoholic potassium hydroxide, haloalkanes lose H-X and form the corresponding alkene. Very strong bases such as KNH2/NH3 convert vic-dihalides (haloalkanes with two halogen atoms on adjacent carbons) into alkynes.
Elimination reactions
opposite sides of double bond
E: Entgegen
same sides (zame zides) of double bond
Z: Zusammen
The general formula for an aliphatic alkene is
CnH2n -- e.g. C2H4 or C3H6
Because of the characteristics of pi-bonds, alkenes have very limited rotation around the double bonds between two atoms. In order for the alkene structure to rotate
the pi-bond would first have to be broken - which would require about 60 or 70 kcal of energy. For this reason alkenes have different chemical properties based on which side of the bond each atom is located.
The relative stability of alkenes may be estimated based on the following concepts:
An internal alkene (the double bond not on the terminal carbon) is more stable than a terminal alkene (the double bond is on a terminal carbon).
Internal alkenes are more stable than terminal alkenes because they are connected to more carbons on the chain. Since a terminal alkene is located at the end of the chain, the double bond is only connected to one carbon, and is called primary (1°). Primary carbons are the least stable. In the middle of a chain, a double bond could be connected to two carbons. This is called secondary (2°). The most stable would be quaternary (4°).
In general, the more and the bulkier the alkyl groups on a sp2-hybridized carbon in the alkene, the more stable that alkene is. A trans double bond is more stable than a cis double bond.
Dehydrohalogenation is a very common method for creating
alkenes. It uses the E2 elimination mechanism that we'll discuss in detail at the end of this chapter. The base used is generally a strong base such as KOH (potassium hydroxide) or NaOCH3 (sodium methoxide). The haloalkane must have a hydrogen and halide 180° from each other on neighboring carbons. If there is no hydrogen 180° from the halogen on a neighboring carbon, the reaction will not take place.
is another method for synthesizing alkenes. The reaction can take place using either sodium iodide in a solution of acetone, or it can be performed using zinc dust in a solution of either heated ethanol or acetic acid.
This reaction can also be performed
The dehalogenation of vicinal dihalides (halides on two neighboring carbons, think "vicinity")
When an alcohol is treated with a strong acid, for example H2SO4, it is converted into an
alkene. The mechanism of this reaction is fairly straight-forward. A lone pair from the alcohol's oxygen attacks a proton (H+) from the acid. This create a hydronium ion which easily leaves the carbon, creating a carbocation. The acid, now deficient a proton, and the carbocation wanting to stabilize and get a bond, a hydrogen from an adjacent bond leaves, without its electron, and the adjacent carbons form a pi bond, creating the alkene.
The underlying mechanism is the E1 mechanism
:"When an unsymmetrical alkene reacts with a hydrogen halide to give an alkyl halide, the hydrogen adds to the carbon of the alkene that has the greater number of hydrogen substituents, and the halogen to the carbon of the alkene with the fewer number of
Markovnikov's Rule (This rule is often compared to the phrase: "The rich get richer and the poor get poorer." Aka, the Carbon with the most Hydrogens gets another Hydrogen and the one with the least Hydrogens gets the halogen)
This means that the nucleophile of the electophile-nucleophile pair is bonded to the position most stable for a carbocation, or partial positive charge in the case of a transition state.
ExamplesCH2 = CH − CH3 + H − Br − > CH3 − CHBr − CH3 Here the Br attaches to the middle carbon over the terminal carbon, because of Markovnikov's rule, and this is called a Markovnikov product.
hydrogen ends up on the more substituted carbon of the double bond. The hydroboration/oxidation reaction is an example of this, as are reactions that are conducted in peroxides.
Anti-Markovnikov addition
are carbon atoms in an organic molecule bearing a positive formal charge. Therefore they are _____ _______. __________have only six electrons in their valence shell making them electron deficient. Thus, they are unstable electrophiles and will react very
The orbitals of carbocations are generally ______ hybridized to that the three full orbitals are arranged in a trigonal planar geometry about the carbon nucleus. The remaining p orbital is empty and will readily accept a pair of electrons from another ato
CH3+ < RCH2+ < R2CH+ < R3C+
Stability of Alkyl Carbocations
Carbocation intermediates are formed in three main types of reactions
additions to pi bonds, unimolecular eliminations, and unimolecular nucleophilic substitution. on bridge head positive never occurs. 3-cyclopropyl carb cation is the most stable carbcation
In general, carbocations will undergo three basic types of reactions:
1. Nucleophile- CaptureCarbocations will react with even mild nucleophiles (such as water) to form a new bond.and formation of carbon free radical
2. Elimination- to form a pi bondCarbons alpha to the carbocation will often lose a proton to form a double (or, in some cases) triple bond from the carbocation. Such a reaction requires only a mild base (e.g. chloride) to remove the proton.
3. Rearrangement- A secondary carbocation may rearrange to form a tertiary carbocation before the ion is stabilized using one of the above-mentioned reactions. Since a cation constitutes a deficiency of electrons, the empty orbitals do not move; rather, a hydrogen atom bonded to a nearby carbon is moved to stabilize the secondary carbocation, but this movement of the hydrogen atom creates a new tertiary carbocation, which is more stable and will be substituted to lead to the final product. See w:carbocation rearrangement.
The addition of BH3 is a concerted reaction in that several bonds are broken and formed at the same time. _________ happens in what's called syn-addition because the boron and one of its hydrogens attach to the same side of the alkene at the same time.
is the addition of two substituents to the same side (or face) of a double bond or triple bond, resulting in a decrease in bond order but an increase in number of substituents. Generally the substrate will be an alkene or alkyne.
Another useful method for creating alcohols from alkenes, is Oxymercuration/Reduction. Like the Hydroboration/Reduction, this too is a two-step process. In the first step,
Unlike Hydroboration/Oxidation, this reaction, follows Markovnikov's rule. The hydrogen is added to the least substituted side and the hydroxyl group is added to the most substituted side.
CH2=CH2 + H2 + catalyst → CH3-CH3
Catalytic hydrogenation of alkenes produce the corresponding alkanes. The reaction is carried out under pressure in the presence of a metallic catalyst.
Most addition reactions to alkenes follow the mechanism of
electrophilic addition. An example is the Prins reaction, where the electrophile is a carbonyl group.
Addition of elementary bromine or chlorine to alkenes yield vicinal dibromo- and dichloroalkanes, respectively.
The decoloration of a solution of bromine in water is an analytical test for the presence of
alkenes: CH2=CH2 + Br2 → BrCH2-CH2Br
The reaction works because the high electron density at the double bond causes a temporary shift of electrons in the Br-Br bond causing a temporary induced dipole. This makes the Br closest to the double bond slightly positive and therefore an electrophile.
Addition of hydrohalic acids like HCl or HBr to alkenes yield the corresponding
an example of this type of reaction: CH3CH=CH2 + HBr → CH3-CHBr-CH3
If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with less hydrogen substituents (Markovnikov's rule).
Addition of a carbene or carbenoid yields the corresponding cyclopropane
Alkenes are oxidized with a large number of oxidizing agents. In the presence of oxygen,
alkenes burn with a bright flame to carbon dioxide and water. Catalytic oxidation with oxygen or the reaction with percarboxylic acids yields epoxides.
Reaction with ozone in ozonolysis leads to the breaking of the double bond, yielding two aldehydes or ketones: R1-CH=CH-R2 + O3 → R1-CHO + R2-CHO + H2O
This reaction can be used to determine the position of a double bond in an unknown alkene.
are hydrocarbons containing carbon-carbon triple bond. They exhibit neither geometric nor optical isomerism.
Alkenes are molecules that consist of carbon and hydrogen atoms where one or more pairs of carbon atoms participate in a double bond, which consists of one sigma (σ) and one pi (π) bond. Alkynes are also molecules consisting of carbon and hydrogen atoms
the alkyne has at least one pair of carbon atoms who have a σ and two π bonds -- a triple bond.
The carbon-carbon triple bond, then, is a bond in which the carbon atoms share an s and two p orbitals to form just one σ and two π bonds between them. This results in a linear molecule with a bond angle of about 180゚. Since we know that double bonds are shorter than single covalent bonds, it would be logical to predict that the triple bond would be shorter still, and this is, in fact, the case.
Alkyne also cycloalkene
is equal to the number of pairs of Hydrogens that must be taken away from the alkane to get the same molecular formula of the compound under investigation. Every π-bond in the molecule increases the index by one.
Index of Hydrogen Deficiency
One double bond 1
1 ring 1
1 double bond and 1 ring 2
2 double bonds 2
1 triple bond 2
1 triple bond + 1 double bond 3
3 double bonds 3
2 double bonds + 1 ring 3
2 triple bonds 4
4 double bonds 4
Index of Hydrogen Deficiency
In speculating as to what the bonding and structure could be with an index of 3 that could mean:
Three double bonds in a non-cyclic structure like hexatriene
Two double bonds in a ring structure like a cyclohexadiene
One triple bond and one double bond in a non-cyclic structure
Clearly the answer cannot be determined from the formula alone, but the formula will give important clues as to a molecule's structure.
In order to synthesize alkynes, one generally starts with
a vicinal or geminal dihalide (an alkane with two halogen atoms attached either next to one another or across from one another). Adding sodium amide (NaNH2) removes the halogens with regiochemistry subject to Zaitsev's Rule, resulting in a carbon-carbon triple bond due to the loss of both halogens as well as two hydrogen atoms from the starting molecule. This is called a double dehydrohalogenation.
Liquid alkynes are non-polar solvents, immiscible with water. Alkynes are, however, more polar than
alkanes or alkenes, as a result of the electron density near the triple bond. Alkynes with a low ratio of hydrogen atoms to carbon atoms are highly combustible. Carbon-carbon triple bonds are highly reactive and easily broken or converted to double or single bonds. Triple bonds store large amounts of chemical energy and thus are highly exothermic when broken. The heat released can cause rapid expansion, so care must be taken when working with alkynes such as acetylene.
One synthetically important property of terminal alkynes is
the acidity of their protons. Whereas the protons in alkanes have pKa's around 60 and alkene protons have pKa's in the mid-40's, terminal alkynes have pKa's of about 25. Substitution of the alkyne can reduce the pKa of the alkyne even further
Alkynes can be hydrated into
a ketone or an aldehyde form. A (Markovnikov) ketone can be created from an alkyne using a solution of aqueous sulfuric acid (H2O/H2SO4) and HgSO4, whereas the anti-Markovnikov aldehyde product requires different reagents and is a multi-step process.
RCCH + H-Br (1 equiv) --> RCBr=CH2
RCCH + H-Br (2 equivs) --> RCBr2CH3
Hydrohalogenation of Alkynes
Alkynes react very quickly and to completion with hydrogen halides. Addition is anti, and follows the Markovnikov Rule.
Adding a halide acid such as HCl or HBr to an alkyne can create a geminal dihalide via a Markovnikov process, but adding HBr in the presence of peroxides results in
Anti-Markovnikov alkenyl bromide product.
Adding diatomic halogen molecules such as Br2 or Cl2 results in 1,2-dihaloalkene, or, if the halogen is added in excess, a 1,1,2,2-tetrahaloalkane.
Adding H2O along with the diatomic halide results in
a halohydrin addition and an α-halo ketone.
2 C2H2 + 5 O2 --> 4 CO2 + 2 H2O
CombustionAlkynes burn in air with a sooty, yellow flame, like alkanes. Alkenes also burn yellow, while alkanes burn with blue flames.
Alkynes can be hydrogenated by adding H2 with a metallic catalyst, such as palladium-carbon or platinum or nickel, which results in
both of the alkyne carbons becoming fully saturated. If Lindlar's catalyst is used instead, the alkyne hydrogenates to a Z enantiomer alkene, and if an alkali metal in an ammonia solution is used for hydrogenating the alkyne, an E enantiomer alkene is the result.
RCCR' + 2 H2 (Pt cat.)--> RCH2CH2R'
alkynes are reduced to alkanes in the presence of an active metal catalyst, such as Pt, Pd, Rh, or Ni in the presence of heat and pressure.
There are two kinds of addition type reactions where a π-bond is broken and atoms are added to the molecule. If the atoms are added on the same side of the molecule then the addition is said to be a _________. If the added atoms are added on opposite sid
"syn" addition, "anti" addition
alkynes can be reduced to cis-alkenes by hydrogen in the presence of Lindlar Pd, i.e. palladium doped with CaSO4 or BaSO4.
In regards to the syn-hydrogenation, anti is
hydrogenation when one hydrogen is added from the top of the pi bond and the other is added from the bottom.
Because of the acidity of the protons of terminal alkynes, they are easily converted into
alkynyl anions in high yield by strong bases.
RCCH + NaNH2 -> RCCNa + NH3 C4H9Li + RCCH -> C4H10 +RCCLi
Alkynes are stronger bases than water, and acetylene (ethyne) is produced in a science classroom reaction of calcium carbide with water.
CaC2 + 2 H20 --> Ca(OH)2 + C2H2
Alkynyl anions are useful in lengthening carbon chains. They react by nucleophilic substitution with alkyl halides.
R-Cl + R'CCNa --> RCCR' + NaCl
The product of this reaction can be reduced to an alkane with hydrogen and a platinum or rhodium catalyst, or an alkene with Lindlar palladium.
are the family of compounds that contain one or more hydroxyl (-OH) groups. Alcohols are represented by the general formula R-OH.
common source for producing alcohols is
from carbonyl compounds. The choice of carbonyl type (ketone, aldehyde, ester, etc) and the type of reaction (Grignard addition or Reduction), will determine the product(s) you will get. Fortunately, there are a number of variations of carbonyls, leading to a number of choices in product.
There are primarily two types of reactions used to create alcohols from carbonyls:
Grignard Addition reactions and Reduction reactions.
The general mechanism of a Grignard reagent reacting with a carbonyl (except esters) involves
the creation of a 6-membered ring transition state. The pi bond of the oxygen attacks a neighboring magnesium bromide which in turn, releases from its R group leaving a carbocation. At the same time, the magnesium bromide ion from another Grignard molecule is attacked by the carbocation and has its magnesium bromide ion stolen (restoring it to its original state as a Grignard reagent). The second molecule's carbocation is then free to attack the carbanion resulting from the vacating pi bond, attaching the R group to the carbonyl.
When a formaldehyde is the target of the Grignard's attack, the result is
a primary alcohol.
When an aldehyde is the target of the Grignard's attack, the result is
a secondary alcohol.
When a ketone is the target of the Grignard's attack, the result is
a tertiary alcohol.
Most acyclic ethers can be prepared using Williamson's synthesis. This involves
reacting an alkoxide with a haloalkane. As stated previously, alkoxides are created by reacting an alcohol with metallic sodium or potassium, or a metal hydride, such as sodium hydride (NaH). To minimize steric hindrance and achieve a good yield, the haloalkane must be a primary haloalkane. This is because the mechanism is SN2, where the oxygen atom does a backside attack on the carbon atom with the halogen atom, causing the halogen atom to leave with its electrons.
You can also use the Williamson synthesis to produce
cyclic ethers. You need a molecule that has a hydroxyl group on one carbon and a halogen atom attached to another carbon. This molecule will then undergo an SN2 reaction with itself, creating a cyclic ether and a halogen anion.
Acyclic ethers can be cleaved by a strong acid, typically HI or HBr, but not HCl. The acid breaks the ether apart into
an alcohol and an alkyl halide (a haloalkane.)
The mechanism used in acidic cleavage of ethers depends on
whether they have primary, secondary, or tertiary groups attached to oxygen. If one of the carbons attached to the central oxygen atom is tertiary, benzylic (contains benzene ring), or allylic (contains carbon-carbon double bond), then the cleavage will occur via an SN1 or an E1 mechanism. The E1 mechanism leads to an alcohol and an alkene instead of an alkyl halide.
are organic compounds which contain and are often actually based on one or more atoms of nitrogen. Structurally resemble ammonia in that the nitrogen can bond up to three hydrogens, but ________ also have additional properties based on their carbon connec
R3N+CH2CH2R' + OH- → R3N + H2C=CHR' + H2O
Hofmann elimination of quaternary ammonium salts Quaternary ammonium salts, upon treatment with a strong base, undergo the Hofmann Elimination.
Nitriles are reduced to amines using
hydrogen in the presence of a nickel catalyst, although acidic or alkaline conditions should be avoided to avoid the possible hydrolysis of the -CN group.
LiAlH4 is more commonly employed for the reduction of nitriles on the laboratory scale.
can also be synthesized by alkylation of ammonia. Haloalkanes react with amines to give a corresponding alkyl-substituted amine, with the release of a halogen acid. Such reactions, which are most useful for alkyl iodides and bromides, are rarely employed
Primary amines
have the nitrogen atom directly connected to an aromatic ring structure. Due to its electron withdrawing properties, the aromatic ring greatly decreases the basicity of the amine - and this effect can be either strengthened or offset depending on what sub
Aromatic amines
Hydrogen bonding significantly influences the properties of primary and secondary amines as well as the protonated derivatives of all amines. Thus the boiling point of amines is
higher than those for the corresponding phosphines (compounds containing phosphorus), but generally lower than the corresponding alcohols. Alcohols, or alkanols, resemble amines but feature an -OH group in place of NR2. Since oxygen is more electronegative than nitrogen, RO-H is typically more acidic than the related R2N-H compound.
Most aliphatic amines display some solubility in water, reflecting their ability to form
hydrogen bonds. Solubility decreases relatively proportionally with the increase in the number of carbon atoms in the molecule - especially when the carbon atom number is greater than six. Aliphatic amines also display significant solubility in organic solvents, especially in polar organic solvents. Primary amines react readily with ketone compounds (such as acetone), however, and most amines are incompatible with chloroform and also with carbon tetrachloride as solvent solutions.
have their lone pair electrons conjugated ("shared") into the benzene ring, so their tendency to engage in hydrogen bonding is somewhat diminished. The boiling points of these molecules are therefore usually somewhat higher than other, smaller amines due
Aromatic amines
The basicity of amines varies by molecule, and it largely depends on:
Like ammonia, amines act as bases and are reasonably strong
The availability of the lone pair of electrons from nitrogen
The electronic properties of the attached substituent groups (e.g., alkyl groups enhance the basicity, aryl groups diminish it, etc.)
The degree of solvation of the protonated amine, which depends mostly on the solvent used in the reaction
The nitrogen atom of a typical amine features a lone electron pair which can bind a hydrogen ion (H+) in order to form an ammonium ion -- R3NH+. The water solubility of simple amines is largely due to the capability for hydrogen bonding that can occur between protons on the water molecules and these lone pairs of electrons.
Acyl chlorides and acid anhydrides react with primary and secondary amines without the presence of heat to form
amides. Tertiary amines cannot be acylated due to the absence of a replaceable hydrogen atom. With the much less active benzoyl chloride, acylation can still be performed by the use of excess aqueous base to facilitate the reaction.
Because amines are basic, they neutralize carboxylic acids to form
the corresponding ammonium carboxylate salts. Upon heating to 200°C, the primary and secondary amine salts dehydrate to form the corresponding amides.
Amines R3N react with strong acids such as hydroiodic acid (HI), hydrobromic acid (HBr) and hydrochloric acid (HCl) to give
ammonium salts R3NH+.
Primary aliphatic amines with nitrous acid give
very unstable diazonium salts which spontaneously decompose by losing N2 to form a carbenium ion. The carbenium ion goes on to produce a mixture of alkenes, alkanols or alkyl halides, with alkanols as the major product. This reaction is of little synthetic importance because the diazonium salt formed is too unstable, even under quite cold conditions.
NaNO2 + HCl → HNO2 + NaCl
Primary amines react with carbonyl compounds to form
imines. Specifically, aldehydes become aldimines, and ketones become ketimines. In the case of formaldehyde (R' = H), the imine products are typically cyclic trimers.
RNH2 + R'2C=O → R'2C=NR + H2O
Secondary amines react with ketones and aldehydes to form
enamines. An enamine contains a C=C double bond, where the second C is singly bonded to N as part of an amine ligand.
R2NH + R'(R"CH2)C=O → R"CH=C(NR2)R' + H2O
______ can be derived from alcohols. The functional group of _______ is R-O-R (instead of R-O-H in an alcohol). ________ can be viewed as a water molecule in which both H atoms are replaced with alkyl groups. ________ may exist in straight chain carbons (
are simply hydrocarbons which contain two double bonds. _______ are intermediate between alkenes and polyenes.
Dienes can divided into three classes:
Unconjugated dienes have the double bonds separated by two or more single bonds.
Conjugated dienes have conjugated double bonds separated by one single bond
Cumulated dienes (cumulenes) have the double bonds sharing a common atom as in a group of compounds called allenes.
Any cyclic compound with 4n+2 pi electrons in the system is
aromatic. The stability of aromatic compounds arises because all bonding orbitals are filled and low in energy.
is a hexagonal ring of six carbon atoms connected to each other through one p-orbital per carbon. Its chemical formula is C6H6, and its structure is a hexagonal ring of carbons sharing symmetrical bonds, with all six hydrogen atoms protruding outwards fro
is a colorless, flammable liquid with a sweet aroma and carcinogenic effects.
An aromatic compound contains a set of covalently-bound atoms with specific characteristics:
1.A delocalized conjugated pi system, most commonly an arrangement of alternating single and double bonds
2.Coplanar structure, with all the contributing atoms in the same plane
3.Contributing atoms arranged in one or more rings
4.A number of pi delocalized electrons that is even, but not a multiple of 4. (This is known as Hückel's rule. Permissible numbers of π electrons include 6, 10, 14, and so on)
5.Special reactivity in organic reactions such as electrophilic aromatic substitution and nucleophilic aromatic substitution
Whereas benzene is aromatic (6 electrons, from 3 double bonds), cyclobutadiene is not, since
the number of π delocalized electrons is 4, which is not satisfied by any n integer value. The cyclobutadienide (2−) ion, however, is aromatic (6 electrons).
Depending on the type of substituent, atoms or groups of atoms may serve to make the benzene ring either more reactive or less reactive. If the atom or group makes the ring more reactive, it is called _______ , and if less, then it is called _________ .
activating, deactivating
Generally, the terms activating and deactivating are in terms of the reactions that fall into the category of
Electrophilic Aromatic Substitution (EAS).
In EAS, a hydroxyl groups is strongly activating, but in Nucleophilic Aromatic Substitution, a hydroxyl group is
strongly deactivating.
In addition to activating or deactivating, all groups and/or substituent atoms on a benzene ring are
-NH2, -NHR, -NRR
very strong ortho/para
-OH, -O-
very strong ortho/para
strong ortho/para
-OCH3, -OR
strong ortho/para
-CH3, -C2H5, -R
weak ortho/para
very weak ortho/para
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