Alkanes
Alkanes are the most simple organic substances made up of carbon and hydrogen only. They have a simple formula of CnH2n +2. In these substances, the 4 valencies of carbon atoms are completed by single bonds to either other carbon atoms or a hydrogen atom.
They are, for that reason known as Saturated Hydrocarbons. Methane (CH4) is the most simple member of this family. Each carbon atom in an alkane is sp3 hybridized and has a tetrahedral geometry.
- 1) Properties of Alkanes
- 2) Structure of Alkanes
- 3) Reactivity of Alkanes
- 4) Inertness of σ-bond
- 5) Uses of Alkanes
- 6) Quest Fest
- 7) Q: Why the boiling point of straight-chain hydrocarbons is greater than their isomeric branch chain hydrocarbons?
- 8) MCQs with Answers
- 9) Frequently Asked Questions (FAQs)
- 10) Summary: Alkanes – Properties, Structure, Reactivity, and Uses
- 11) You may also like to learn:
Properties of Alkanes
1.Alkanes containing up to 4 carbon atoms are colorless, odorless gases while pentane to heptadecane (C5 to C17) are colorless, odorless liquids. The greater members from C18 onwards are waxy solids which are also colorless and odorless.
- Alkanes are non-polar or very weakly polar and are insoluble in polar solvents like water, however soluble in non-polar solvents like benzene, ether, carbon tetrachloride, and so on.
- Their physical constants like boiling points, melting points, density, etc. increase with the increase in the number of carbon atoms, whereas solubility reduces with an increase in molecular mass. The boiling point increases by 20 to 30 ° C for the addition of each CH2 group to the molecule. The boiling points of alkanes having branched-chain structures are lower than their isomeric typical chain alkanes, e.g., n-butane has a higher boiling point-0.50 C than isobutane (-1 1.7 ° C).
- The melting points of alkanes also increase with the increase in molecular mass but this increase is not so regular.
Structure of Alkanes
Alkanes have the general formula CnH2n +2. For instance, an alkane with 2 (n) carbon atoms, will have 6 (2n + 2) hydrogen atoms. Their surrounding atoms are connected with sigma bonds and form tetrahedral centers around the carbon atoms. As these bonds are all single, there is free rotation around all connections.
Each carbon atom has 4 bonds (either C-H or C-C bonds), and each hydrogen atom is signed up with a carbon atom (H-C bonds). A series of linked carbon atoms is referred to as the carbon skeleton or carbon backbone. The number of carbon atoms is used to specify the size of the alkane (e.g., C2-alkane).
An alkyl group, normally abbreviated with the symbol R, is a functional group or side-chain that, like an alkane, consists solely of single-bonded carbon and hydrogen atoms; for example, R might represent a methyl or ethyl group. An alkyl group is a piece of a particle with the general formula (CH3) n, where n is an integer. For example, a methyl group (CH3) is a fragment of a methane molecule (CH4). In this example, n= 1.
Reactivity of Alkanes
The alkanes are also called paraffins (Latin: parum = little, affins = affinity) under regular conditions and are inert towards acids, alkalis, oxidizing, and decreasing agents. Nevertheless, under ideal conditions, alkanes do undergo two kinds of reactions.
- Substitution Reactions
- Thermal and Catalytic Reactions
These reactions happen at high temperatures or on the absorption of light energy through the development of extremely reactive complimentary radicals. The unreactivity of alkanes under typical conditions might be discussed based on the non-polarity of the bonds forming them.
The electronegativity values of carbon (2.5) and hydrogen (2.1) do not differ significantly and the bonding electrons between C-H and C-C are equally shared making them practically nonpolar. Given this, the ionic reagents such as acids, alkalies, oxidizing agents, etc. find no reaction site in the alkane particles to which they could be connected.
Inertness of σ-bond
The unreactivity of alkanes can also be discussed based on the inertness of a σ-bond. In a σ-bond, the electrons are firmly held between the nuclei which makes it a stable bond. A great deal of energy is needed to break it. Furthermore, the electrons present in a σ-bond can neither attack on any electrophile nor a nucleophile can attack them. Both these facts make alkanes less reactive.
Uses of Alkanes
Alkanes are really versatile and are being used as solvents, heating oils, fuels, in fat synthesis, in the synthesis of fats by air oxidation, in the manufacture of albumen, in the change to olefins, etc.
- Methane is used:
- (i) as a fuel and as illuminating gas.
- (ii) for the preparation of methyl chloride, dichloromethane, chloroform, and carbon tetrachloride.
- (iii) for the industrial preparation of methyl alcohol, formaldehyde, and hydrogen cyanide.
- (iv) for the preparation of carbon black utilized in paints, printing inks, and auto tires.
- (v) is utilized to produce urea fertilizer.
- Ethane
Ethane is being used as heating fuel. In addition, ethane is made used of to prepare ethylene by pyrolysis or acetaldehyde and acetic acid by catalytical oxidation.
- Propane
Propane is an essential and basic raw material in petrochemistry.
- Butane
It finds use as liquid gas (LPG) in laboratories and households for heating and combustion functions.
- Pentane
In addition to being used as a solvent, n-pentane is utilized as a lathering agent in the preparation of phenolic resins and polystyrene, as a propellant for aerosol sprays, as a filling of low-temperature thermometers, and as a reference compound in gas chromatography.
- Hexane
Hexane is used for the extraction of oils and fats, as a solvent and response medium in the preparation of plastics and synthetic rubber during the polymerization procedure, and as a dilutant for fast-drying lacquers, printing inks, and glues.
- Heptane
Heptane functions as a solvent in the laboratory and for fast-drying lacquers and glues.
- Octane
Octane is utilized for the aromatization of xylenes and ethylbenzene but mainly as a solvent and in azeotropic distillations.
- Nonane
Nonane is used for the preparation of tensides (detergents) and as a carrier in distillation processes.
Quest Fest
Q: Why the boiling point of straight-chain hydrocarbons is greater than their isomeric branch chain hydrocarbons?
Ans: The boiling point of a molecule depends upon the strength of intermolecular forces. In straight-chain hydrocarbons, the London dispersion forces (intermolecular forces) are strong because the molecules are close to each other and the area of interaction between the two molecules is large.
In isomeric branched-chain hydrocarbons, the London dispersion forces (intermolecular forces) are weak because the distance between the molecule increases and the area of interaction decreases due to branching.
Hence, the boiling point of straight-chain hydrocarbons is greater than branched-chain hydrocarbons.
MCQs with Answers
- What is the general formula for alkanes?
- A) CnH2n
- B) CnH2n+2
- C) CnHn
- D) CnHn+2
- Answer: B
- How are the carbon atoms in alkanes connected?
- A) Double bonds
- B) Triple bonds
- C) Single bonds
- D) Ionic bonds
- Answer: C
- Which member is the simplest in the alkane family?
- A) Ethane
- B) Methane
- C) Propane
- D) Butane
- Answer: B
- What is the molecular formula of methane?
- A) CH4
- B) C2H6
- C) C3H8
- D) C4H10
- Answer: A
- Why are alkanes called saturated hydrocarbons?
- A) They have double bonds
- B) They have triple bonds
- C) They have only single bonds
- D) They have ionic bonds
- Answer: C
- Which statement about alkane solubility is correct?
- A) They are soluble in water
- B) They are soluble in polar solvents
- C) They are insoluble in non-polar solvents
- D) They are insoluble in polar solvents
- Answer: D
- What is the trend of boiling points in alkanes with an increase in carbon atoms?
- A) Decrease
- B) Remain constant
- C) Irregular
- D) Increase
- Answer: D
- How does branching in alkanes affect boiling points?
- A) Increases
- B) Decreases
- C) No effect
- D) Random effect
- Answer: B
- What type of geometry do carbon atoms have in alkanes?
- A) Linear
- B) Trigonal planar
- C) Tetrahedral
- D) Octahedral
- Answer: C
- What is the reaction type that alkanes undergo at high temperatures?
- A) Addition reactions
- B) Substitution reactions
- C) Combustion reactions
- D) Oxidation reactions
- Answer: B
- Why are alkanes considered inert under normal conditions?
- A) Due to high polarity
- B) Due to low boiling points
- C) Due to high reactivity
- D) Due to non-polarity
- Answer: D
- What is the primary reason for the unreactivity of alkanes?
- A) High electronegativity
- B) Low boiling points
- C) Inertness of σ-bond
- D) Strong London forces
- Answer: C
- What is an alkyl group?
- A) A functional group with double bonds
- B) A side-chain with single-bonded carbon and hydrogen atoms
- C) An isomer of alkane
- D) A cyclic hydrocarbon
- Answer: B
- How are alkanes used in the synthesis of fats?
- A) In heating oils
- B) In fuel production
- C) In air oxidation
- D) In the manufacture of albumen
- Answer: C
- What is the primary use of propane?
- A) Heating fuel
- B) Solvent
- C) Raw material in petrochemistry
- D) LPG in households
- Answer: A
- Which alkane is used as a lathering agent in the preparation of resins?
- A) Methane
- B) Butane
- C) Pentane
- D) Hexane
- Answer: C
- Why does the boiling point of straight-chain hydrocarbons exceed that of branched-chain hydrocarbons?
- A) Due to stronger London dispersion forces
- B) Due to higher polarity
- C) Due to lower molecular mass
- D) Due to weaker intermolecular forces
- Answer: A
Frequently Asked Questions (FAQs)
- What are alkanes, and what is their basic chemical formula?
- Alkanes are the simplest organic compounds composed of carbon and hydrogen. Their basic formula is CnH2n+2.
- Which is the simplest alkane, and what is its molecular formula?
- Methane is the simplest alkane with the molecular formula CH4.
- How are the carbon atoms in alkanes connected to each other?
- Carbon atoms in alkanes are connected by single bonds, forming a tetrahedral geometry.
- What physical states do alkanes exhibit based on the number of carbon atoms?
- Alkanes with up to 4 carbon atoms are colorless, odorless gases. Those from C5 to C17 are colorless, odorless liquids, while those with more than C17 are waxy, colorless, and odorless solids.
- Are alkanes polar or non-polar, and in which solvents are they soluble?
- Alkanes are non-polar or very weakly polar. They are insoluble in polar solvents like water but soluble in non-polar solvents like benzene and ether.
- How does the boiling point of alkanes change with the increase in the number of carbon atoms?
- Boiling points of alkanes increase with an increase in the number of carbon atoms. Additionally, branching in alkanes reduces boiling points.
- What is the general formula for alkanes, and how is their structure described?
- The general formula is CnH2n+2. Their structure involves single bonds (sigma bonds) with tetrahedral centers around carbon atoms, allowing free rotation.
- Why are alkanes considered inert under normal conditions?
- Alkanes are inert due to their non-polar nature, making them unreactive towards acids, alkalis, oxidizing, and reducing agents under regular conditions.
- What types of reactions do alkanes undergo under specific conditions?
- Alkanes undergo substitution reactions and thermal/catalytic reactions under high temperatures or light absorption, leading to the formation of highly reactive free radicals.
- How can the unreactivity of alkanes be explained in terms of σ-bond inertness?
- The inertness of alkanes is attributed to the stability and inert nature of σ-bonds, requiring significant energy to break. The electrons in σ-bonds are not readily available for attack by electrophiles or nucleophiles.
- What are some common uses of alkanes in various industries?
- Alkanes are versatile and used as solvents, heating oils, fuels, in fat synthesis, air oxidation of fats, manufacture of albumen, and conversion to olefins.
- Can you provide examples of specific alkanes and their uses?
- Methane is used as a fuel, illuminating gas, and in the production of various chemicals. Ethane is used as heating fuel, propane is vital in petrochemistry, and butane serves as a household heating gas. Other alkanes like pentane, hexane, and octane find applications in solvents, reactions, and distillation processes.
- Why do straight-chain hydrocarbons have a higher boiling point than their branched isomers?
- The boiling point depends on intermolecular forces. Straight-chain hydrocarbons have stronger London dispersion forces due to closer proximity, leading to a larger interaction area compared to branched isomers, resulting in a higher boiling point.
Summary: Alkanes – Properties, Structure, Reactivity, and Uses
Alkanes, the simplest organic compounds consisting of carbon and hydrogen, exhibit unique properties, structures, and applications. With a fundamental formula of CnH2n+2, they are classified as saturated hydrocarbons. Starting with methane (CH4), each carbon atom in alkanes is sp3 hybridized, forming a tetrahedral geometry.
Properties of alkanes vary based on the number of carbon atoms, with smaller ones being colorless gases and larger ones becoming waxy solids. Alkanes are non-polar or weakly polar, insoluble in water but soluble in non-polar solvents. Physical constants like boiling and melting points increase with carbon atom count.
The structure of alkanes is defined by the general formula CnH2n+2, and their sigma bonds allow free rotation around connections. Alkyl groups (represented by R) are functional groups that play a crucial role in defining alkane structures.
The reactivity of alkanes is characterized by their inertness under normal conditions, resisting acids, alkalis, and oxidizing or reducing agents. However, under specific conditions, they undergo substitution reactions and thermal/catalytic reactions, leading to the formation of reactive radicals.
The unreactivity of alkanes can be explained by the non-polarity of their bonds and the inertness of sigma (σ) bonds, which require substantial energy to break. This inertness makes alkanes less reactive in various chemical environments.
Alkanes find versatile applications, serving as solvents, heating oils, fuels, and playing a role in fat synthesis. Methane, ethane, propane, and other members contribute to industrial processes, such as the production of chemicals like methyl alcohol and formaldehyde. The Quest Fest section addresses a common question about the boiling points of straight-chain and branched-chain hydrocarbons, emphasizing intermolecular forces.
In conclusion, alkanes, with their distinct properties and applications, form a foundational aspect of organic chemistry, influencing various industries and processes.