Haloalkanes Preparation and Properties

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Questions and Answers

What happens to the carbon-halogen bond length as you move from C–F to C–I?

It first increases, then decreases.

It decreases.

It increases. (correct)

It remains constant.

Which of the following statements correctly describes the nature of carbon-halogen bonds in alkyl halides?

They are non-polar due to similar electronegativities.

The carbon atom bears a partial positive charge. (correct)

The carbon atom bears a partial negative charge.

The halogen atom bears a partial positive charge.

Which halogen alkane has the highest bond enthalpy among those listed?

CH3–I

CH3–Br

CH3–Cl

CH3–F (correct)

What method is preferred for preparing alkyl halides from alcohols?

Using thionyl chloride. Signup and view all the answers

Which of the following best describes the dipole moment of CH3–Br?

Lower than CH3–I. Signup and view all the answers

What is the correct order of boiling points for alkyl halides with the same alkyl group?

RI > RBr > RCl > RF Signup and view all the answers

How does the boiling point of isomeric haloalkanes change with branching?

It decreases with increased branching. Signup and view all the answers

Which statement about the melting points of para-isomers compared to ortho- and meta-isomers is correct?

Para-isomers have higher melting points due to better symmetry. Signup and view all the answers

What trend is observed regarding the density of haloalkanes?

Density increases with an increase in halogen atomic mass. Signup and view all the answers

How soluble are haloalkanes in water?

Very slightly soluble in water. Signup and view all the answers

Which molecule is considered achiral due to the absence of an asymmetric carbon atom?

Propan-2-ol Signup and view all the answers

What characterizes a chiral molecule in relation to its mirror image?

It contains at least one asymmetric carbon atom. Signup and view all the answers

Which of the following is an example of a chiral molecule?

Butan-2-ol Signup and view all the answers

What happens when the mirror image of butan-2-ol is rotated by 180°?

It remains non-superimposable on its original structure. Signup and view all the answers

Which of the following is false about achiral molecules?

They can exist in chiral forms depending on their environment. Signup and view all the answers

What is the primary effect that halogens have on stabilizing carbocations?

Resonance effect Signup and view all the answers

Which reaction is referred to when an alkyl halide and aryl halide are treated with sodium in dry ether to form an alkylarene?

Wurtz-Fittig reaction Signup and view all the answers

Which of the following statements is true regarding the reactivity of haloalkanes and haloarenes?

Reactivity is controlled by the stronger inductive effect. Signup and view all the answers

What are carbon compounds containing more than one halogen atom usually referred to as?

Polyhalogen compounds Signup and view all the answers

In the context of electrophilic aromatic substitution, what does the resonance effect tend to do?

Oppose the inductive effect Signup and view all the answers

What was a significant problem associated with the extensive use of DDT after World War II?

Resistance developed in many insect species. Signup and view all the answers

How does DDT accumulate in animals?

It builds up in fatty tissues over time. Signup and view all the answers

Which method is NOT used to prepare alkyl halides?

Electrophilic substitution to arenes. Signup and view all the answers

What is a characteristic of the carbon-halogen bond in alkyl halides?

The bond is polarized due to the differing electronegativities. Signup and view all the answers

What is the primary reason for the ban of DDT in the United States in 1973?

It caused significant environmental toxicity. Signup and view all the answers

Study Notes

C-X Bond

Halogen atoms are more electronegative than carbon, making the carbon-halogen bond polar.
The carbon atom bears a partial positive charge, and the halogen atom bears a partial negative charge.
As you move down the periodic table, halogens become larger.
Fluorine is the smallest, and iodine is the largest.
This causes the carbon-halogen bond length to increase from C-F to C-I.

Methods of Preparation of Haloalkanes

The hydroxyl group of an alcohol can be replaced by a halogen using concentrated halogen acids, phosphorus halides, or thionyl chloride.
Thionyl chloride is preferred because it produces alkyl halides along with SO2 and HCl gases.
These gases escape, making the reaction produce pure alkyl halides.
For the same alkyl group, the boiling points of alkyl halides follow this order: RI > RBr > RCl > RF.
This is due to the increasing size and mass of the halogen atom, leading to stronger van der Waals forces.
Isomeric haloalkanes exhibit decreasing boiling points with increasing branching.
Dihalobenzenes have similar boiling points, but para-isomers have higher melting points due to their symmetrical structure fitting better in the crystal lattice.

Density

Bromo, iodo, and polychloro derivatives of hydrocarbons are denser than water.
Density increases with the number of carbon and halogen atoms, and with the atomic mass of the halogen atoms.

Solubility

Haloalkanes have very low solubility in water.

Chirality

Asymmetric carbons have four different groups attached and are chiral (non-superimposable mirror images).
Propan-2-ol does not have an asymmetric carbon, so it is not chiral, as the four groups attached to the carbon are not all different.
Butan-2-ol has an asymmetric carbon and four different groups, making it chiral.

Resonance and Inductive Effects

Resonance and inductive effects influence the reactivity of aromatic halides.
Halogens stabilize carbocations through resonance, particularly at ortho and para positions.
The inductive effect is stronger than resonance, leading to net electron withdrawal and deactivation.
For ortho and para positions, the resonance effect counteracts some of the inductive effect, making the deactivation less significant.

Reactions with Metals

Wurtz-Fittig reaction: A mixture of alkyl halide and aryl halide reacts with sodium in dry ether to form an alkylarene.
Fittig reaction: Aryl halides react with sodium in dry ether to form a compound with two joined aryl groups.

Polyhalogen Compounds

Compounds with more than one halogen atom are called polyhalogen compounds.
DDT is an example of a polyhalogen compound with significant industrial and agricultural applications.
DDT's effectiveness against malaria-carrying mosquitoes and typhus-carrying lice led to its widespread use after World War II.
However, DDT's extensive use led to problems, such as resistance in insects, toxicity to fish, and bioaccumulation in animal fatty tissues.
DDT was banned in the United States in 1973.

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Description

Explore the interesting world of haloalkanes with this quiz. Learn about the polarity of carbon-halogen bonds, the methods to prepare haloalkanes, and the relationship between boiling points and halogen size. Test your knowledge on key concepts and chemical properties.