Haloalkanes and Haloarenes

What is the general definition of a haloalkane?

An aliphatic hydrocarbon in which one or more hydrogen atoms have been replaced by halogen atoms.

What is the general definition of a haloarene?

An aromatic hydrocarbon in which one or more hydrogen atoms have been replaced by halogen atoms.

What is the hybridization of the carbon atom to which a halogen is attached in a haloalkane?

The halogen atom is attached to an $sp^3$ hybridized carbon atom.

What is the hybridization of the carbon atom to which a halogen is attached in a haloarene?

The halogen atom is attached to an $sp^2$ hybridized carbon atom of an aryl group.

What type of halide is formed when a halogen atom is bonded to an $sp^3$-hybridized carbon atom adjacent to a carbon-carbon double bond?

An allylic halide.

What type of halide is formed when a halogen atom is bonded to an $sp^3$-hybridized carbon atom which is attached to an aromatic ring?

A benzylic halide.

What type of halide is formed when a halogen atom is bonded to an $sp^2$-hybridized carbon atom of a carbon-carbon double bond?

A vinylic halide.

What distinguishes a primary (1°), secondary (2°), and tertiary (3°) alkyl halide?

The classification is based on the nature of the carbon atom to which the halogen is attached (primary, secondary, or tertiary).

What common name system is used for dihalogen derivatives of benzene, and what is used in the IUPAC system?

The common system uses prefixes o-, m-, p-, while the IUPAC system uses numerals like 1,2; 1,3; and 1,4.

What are geminal dihalides (gem-dihalides)?

Dihalo-compounds where both halogen atoms are present on the same carbon atom.

What are vicinal dihalides (vic-dihalides)?

Dihalo-compounds where halogen atoms are present on adjacent carbon atoms.

Why is the carbon-halogen (C-X) bond in an alkyl halide polarized?

Because halogen atoms are more electronegative than carbon, leading to a partial positive charge on the carbon and a partial negative charge on the halogen.

How does the C-X bond length and bond enthalpy change as you go down the halogen group from F to I?

The bond length increases (C-F to C-I), and the bond enthalpy decreases.

What reagent is preferred for converting alcohols to alkyl halides because its byproducts are escapable gases?

Thionyl chloride ($SOCl2$), which produces $SO2$ and $HCl$ gases.

What is the order of reactivity of alcohols (1°, 2°, 3°) with a given haloacid like HCl?

The order of reactivity is $3° > 2° > 1°$.

Why can't aryl halides be prepared from phenols using methods like reaction with HCl?

The carbon-oxygen bond in phenols has a partial double bond character due to resonance, making it stronger and difficult to break.

What is the outcome of the free radical halogenation of alkanes, and why is it often not a preferred synthetic method?

It gives a complex mixture of isomeric mono- and polyhaloalkanes, resulting in a low yield for any single compound.

According to Markovnikov's rule, what is the major product when an alkene like propene reacts with a hydrogen halide like HBr?

The negative part of the addendum (Br-) attaches to the carbon atom that has fewer hydrogen atoms.

The reaction of an alkyl chloride/bromide with NaI in dry acetone to produce an alkyl iodide is known as the _ reaction.

Finkelstein reaction. Example: $R-Cl + NaI \xrightarrow{dry~acetone} R-I + NaCl(s)$.

The synthesis of an alkyl fluoride by heating an alkyl chloride/bromide with a metallic fluoride like AgF or $Hg2F2$ is called the _ reaction.

Swarts reaction. Example: $CH3-Br + AgF \rightarrow CH3-F + AgBr$.

What type of catalyst is required for the electrophilic substitution of arenes with chlorine or bromine to produce aryl halides?

A Lewis acid catalyst, such as iron ($Fe$) or iron(III) chloride ($FeCl_3$).

The reaction that converts a primary aromatic amine into a diazonium salt, which is then reacted with cuprous chloride or bromide to form an aryl halide, is called the _.

Sandmeyer's reaction.

How are para-isomers and ortho-isomers of dihalobenzenes typically separated?

They are easily separated due to the large difference in their melting points.

Why are para-isomers of dihalobenzenes generally higher melting than their ortho- and meta-isomers?

The symmetry of para-isomers allows them to fit better into the crystal lattice, leading to stronger intermolecular forces.

Why are haloalkanes only slightly soluble in water despite being polar?

The energy required to break hydrogen bonds between water molecules is greater than the energy released when new, weaker attractions form between haloalkanes and water.

A species that is electron-rich and attacks an electron-deficient part of a substrate is known as a(n) _.

Nucleophile ($Nu^-$).

A species that lacks electrons and is attacked by an electron-rich species is known as a(n) _.

Electrophile ($E^+$).

Nucleophiles like the cyanide ($CN^−$) and nitrite ($NO2^−$) groups, which can attack through two different atoms, are called nucleophiles.

Ambident nucleophiles.

Why does KCN react with haloalkanes to form alkyl cyanides, while AgCN forms isocyanides?

KCN is ionic, providing free $CN^−$ ions where attack occurs through carbon; AgCN is covalent, so attack occurs through the lone pair on the nitrogen atom.

What does 'bimolecular' in Substitution Nucleophilic Bimolecular ($S_N2$) signify?

It signifies that the rate of the reaction depends on the concentration of both reactants (the haloalkane and the nucleophile).

Describe the mechanism of an $S_N2$ reaction in one sentence.

It is a single-step reaction where the nucleophile attacks from the side opposite to the leaving group (backside attack), causing a simultaneous bond formation and bond breaking.

What is the stereochemical outcome of an $S_N2$ reaction at a chiral center?

The reaction proceeds with a complete inversion of configuration, often called a Walden inversion.

What is the order of reactivity for alkyl halides in an $S_N2$ reaction?

Primary halide > Secondary halide > Tertiary halide ($1° > 2° > 3°$), due to increasing steric hindrance.

What does 'unimolecular' in Substitution Nucleophilic Unimolecular ($S_N1$) signify?

It signifies that the rate of the reaction depends only on the concentration of one reactant (the haloalkane).

What is the first and rate-determining step of an $S_N1$ reaction?

The slow cleavage of the C-X bond to form a planar carbocation intermediate.

What is the stereochemical outcome of an $S_N1$ reaction at a chiral center?

The reaction results in racemization (formation of a nearly 50:50 mixture of retention and inversion products) because the planar carbocation can be attacked from either side.

What is the order of reactivity for alkyl halides in an $S_N1$ reaction?

Tertiary halide > Secondary halide > Primary halide ($3° > 2° > 1°$), due to the greater stability of the carbocation intermediate.

What type of solvent favors $S_N1$ reactions?

Polar protic solvents (like water, alcohol) are favored because they can solvate and stabilize both the carbocation intermediate and the leaving group.

What type of solvent favors $S_N2$ reactions?

Polar aprotic solvents (like acetone, DMF, DMSO) are favored as they solvate the cation but not the nucleophile, leaving it 'bare' and more reactive.

Why do allylic and benzylic halides show high reactivity towards the $S_N1$ reaction?

The carbocation intermediate formed is stabilized through resonance.

What term describes compounds that can rotate the plane of plane-polarized light?

Optically active compounds.

What is a chiral molecule?

A molecule that is non-superimposable on its mirror image, often containing an asymmetric carbon (a carbon bonded to four different groups).

Stereoisomers that are non-superimposable mirror images of each other are called _.

Enantiomers.

What is a racemic mixture and what is its net optical rotation?

A mixture containing two enantiomers in equal proportions (50:50), which results in zero net optical rotation.

In an elimination reaction of a haloalkane, from which carbon is the hydrogen atom removed?

A hydrogen atom is removed from the β-carbon (the carbon atom adjacent to the α-carbon which bears the halogen).

State Zaitsev's (or Saytzeff's) rule for elimination reactions.

In dehydrohalogenation reactions, the preferred product is the alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms (the more substituted alkene).

The reaction of two molecules of an alkyl halide with sodium metal in dry ether to form a new alkane with double the number of carbon atoms is called the _.

Wurtz reaction. Example: $2CH3-Br + 2Na \xrightarrow{dry~ether} CH3-CH_3 + 2NaBr$.

What is a Grignard reagent and how is it prepared?

It is an organometallic compound with the formula $RMgX$, prepared by reacting a haloalkane with magnesium metal in dry ether.

Why must Grignard reagents be prepared under anhydrous (dry) conditions?

They are highly reactive and will react with any source of a proton, such as water, to give a hydrocarbon, destroying the reagent.

What are two reasons why aryl halides are extremely less reactive towards nucleophilic substitution reactions compared to haloalkanes?

1. Resonance effect gives the C-X bond partial double-bond character. 2. The carbon in the C-X bond is $sp^2$ hybridized, making the bond shorter and stronger.

How does the presence of an electron-withdrawing group (like $-NO_2$) at the ortho- and para-positions affect the reactivity of haloarenes towards nucleophilic substitution?

It greatly increases the reactivity by withdrawing electron density from the ring, which facilitates nucleophilic attack and stabilizes the negatively charged intermediate (Meisenheimer complex).

The industrial process for converting chlorobenzene to phenol using aqueous NaOH at 623 K and 300 atm is known as the _.

Dow process.

Why is the halogen atom on a haloarene considered ortho, para-directing yet deactivating for electrophilic substitution?

The strong -I (inductive) effect deactivates the ring, but the +R (resonance) effect increases electron density at the ortho and para positions, directing the electrophile there.

The reaction of an alkyl halide and an aryl halide with sodium in dry ether to form an alkylarene is known as the _.

Wurtz-Fittig reaction.

The reaction of two molecules of an aryl halide with sodium in dry ether to form a biaryl compound is known as the _.

Fittig reaction. Example: $2C6H5Br + 2Na \xrightarrow{dry~ether} C6H5-C6H5 + 2NaBr$.

What is a primary use of dichloromethane ($CH2Cl2$)?

It is widely used as a solvent, particularly as a paint remover and in the manufacture of drugs.

Why is chloroform ($CHCl_3$) stored in closed, dark-colored bottles that are completely filled?

To prevent its slow oxidation by air in the presence of light, which forms the extremely poisonous gas phosgene ($COCl_2$).

What major environmental problem is associated with carbon tetrachloride ($CCl_4$) and Freons?

When released into the atmosphere, they rise to the stratosphere and cause depletion of the ozone layer.

What is DDT, and for what discovery was Paul Muller awarded the Nobel Prize?

DDT (p,p’-Dichlorodiphenyltrichloroethane) is a chlorinated organic insecticide, and Muller discovered its effectiveness as an insecticide.

Why was the use of DDT banned in many countries?

Due to its chemical stability, fat solubility, and resistance to metabolism, it builds up in fatty tissues (bioaccumulation) and is toxic to wildlife, especially fish.

Compounds of methane and ethane containing chlorine and fluorine atoms, which are used as refrigerants and propellants, are collectively known as _.

Freons.

In the reaction of an alcohol with HX to form a haloalkane, what is the purpose of the $H^+$?

The $H^+$ protonates the alcohol's hydroxyl group, converting it into $H_2O$, which is a much better leaving group.

What is the key intermediate in an E1 elimination reaction?

A carbocation, similar to an $S_N1$ reaction.

What is the key requirement for the leaving group and the β-hydrogen in an E2 elimination reaction?

They must be in an anti-periplanar conformation (in the same plane but on opposite sides of the C-C bond).

How does a bulky nucleophile/base favor elimination over substitution?

A bulky base finds it sterically easier to abstract a proton from the periphery of the molecule (elimination) rather than attacking the electrophilic carbon (substitution).

What is solvolysis?

A nucleophilic substitution reaction where the solvent itself acts as the nucleophile, which typically follows an $S_N1$ pathway.

What term describes the preservation of the spatial arrangement of bonds to a stereocenter during a chemical reaction?

Retention of configuration.

Vinylic halides and aryl halides do not typically undergo $SN1$ or $SN2$ reactions. Why not?

The C-X bond has double bond character, and the resulting carbocation ($sp$-hybridized for vinylic) would be highly unstable.

In electrophilic substitution of chlorobenzene, what are the major products formed during nitration with conc. $HNO3$ and conc. $H2SO_4$?

The major products are 1-chloro-2-nitrobenzene (ortho) and 1-chloro-4-nitrobenzene (para).

What is the reaction called when chlorobenzene is reacted with an alkyl chloride in the presence of anhydrous $AlCl_3$?

Friedel-Crafts alkylation.