9.2.3.2 - Chemical properties
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Inertness and Substitution Reactions
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Good morning, class! Today we're going to discuss the unique chemical properties of alkanes, which are saturated hydrocarbons. Who can remind me what it means for a molecule to be saturated?
It means all the carbon bonds are single bonds with no double or triple bonds!
Exactly! Because of this structure, alkanes are usually quite stable. They don't interact readily with acids or bases. However, they can undergo substitution reactions, particularly with halogens. Can anyone tell me what a halogen is?
Halogens are elements like chlorine, bromine, and iodine.
Precisely! So, when we expose alkanes to halogens under certain conditions like UV light or high temperatures, substitution can occur. What products do you think this reaction produces?
I think it produces halogenated hydrocarbons, like chloromethane from methane and chlorine.
Spot on! Remember, this process occurs through a free radical mechanism consisting of three stages: initiation, propagation, and termination.
Can you give us an example of what happens in the initiation stage?
Certainly! During initiation, the chlorine molecules break apart into free radicals when exposed to light. These radicals then can react with methane, leading to chlorination. Let's remember, substitution reactions are vital in organic synthesis.
To summarize today’s session: Alkanes are inert, but they can engage in substitution reactions with halogens under specific conditions, producing valuable halogenated compounds.
Combustion of Alkanes
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Welcome back, class! Let's move onto combustion reactions of alkanes. Can someone explain what combustion is?
It's a chemical reaction that typically occurs between a fuel and oxygen, releasing energy!
Exactly right! When we burn alkanes in oxygen, we primarily get carbon dioxide and water as products. What is the significance of this reaction in our lives?
It creates energy for heating and powering our vehicles!
Yes! However, if the supply of oxygen is limited, incomplete combustion can occur, leading to the formation of carbon monoxide or even particulate matter. This is important in understanding fuel efficiency and pollution. What can you infer from the energy output of completely combusting an alkane?
It must produce a lot of energy, which is why it's used as fuel!
Exactly! Remember the general combustion equation for any alkane is CnH2n+2 + O2 → CO2 + H2O. Can anyone derive the combustion reaction of butane (C4H10)?
Sure! It should be: 2C4H10 + 13O2 → 8CO2 + 10H2O.
Well done! In conclusion, combustion provides energy, but we need to pay attention to the complete vs. incomplete combustion for environmental safety.
Controlled Oxidation and Other Reactions
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Alright, class! Now let's dive into how alkanes can go through controlled oxidation to yield various organic products. Who can explain what we mean by controlled oxidation?
I think it means that the reaction conditions are carefully chosen to limit what products are formed.
Precisely! When alkanes undergo controlled oxidation, we can produce alcohols like methanol or even acids through specific reagents like potassium permanganate. What happens if we over-oxidize?
We could end up breaking down the molecule too much, turning it into carbon dioxide and water instead.
Exactly! This principle allows us to synthesize valuable compounds. Additionally, alkanes can undergo isomerization to form branched alkanes which often have more favorable physical properties. Can anyone tell me a situation where isomerization is useful?
In fuel production, branched alkanes are usually more efficient and burn better!
Correct! And don't forget about aromatization, where longer alkanes can be converted into aromatic hydrocarbons like benzene, which are essential for industrial chemicals. In summary, controlled conditions can lead to various products when working with alkanes.
Introduction & Overview
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Quick Overview
Standard
The chemical properties of alkanes primarily include their resistance to reaction under normal conditions, allowing them to function as stable compounds often engaging in substitution reactions (like halogenation) under specific circumstances. Alkanes mainly undergo combustion, producing carbon dioxide and water, while their inertness makes them useful in various industrial applications.
Detailed
In this section, we explore the chemical properties of alkanes, which are characterized by the absence of reactivity with acids, bases, and oxidizing agents under mild conditions. Alkanes are generally stable and exhibit inert behavior, making them suitable for a wide range of applications. Key reactions include:
- Substitution Reactions: Alkanes participate in halogenation where hydrogen atoms can be replaced by halogen atoms under high temperatures or UV light. The process follows a free radical mechanism that includes initiation, propagation, and termination phases.
- Example: Methane reacts with chlorine to yield chloromethane and hydrogen chloride.
- Important: The rate of halogenation is influenced by the structure of the alkane, with tertiary carbons being more reactive than secondary or primary.
- Combustion Reactions: Alkanes combust in the presence of oxygen to produce carbon dioxide and water, releasing a significant amount of energy in the form of heat.
- Complete combustion: Alkanes produce carbon dioxide and water.
- Incomplete combustion: Produces carbon monoxide or soot.
- Controlled Oxidation: Alkanes can undergo oxidation under controlled conditions leading to products such as alcohols or acids, especially using permanganate and chromate ions.
- Isomerization: Long-chain alkanes can convert to branched isomers under specific catalytic conditions, optimizing their properties for fuels.
- Aromatization: Alkanes can be converted to aromatic hydrocarbons, reinforcing their significance in organic chemistry.
Understanding these reactions is crucial for utilizing alkanes efficiently in both energy production and as precursors for the synthesis of other organic compounds.
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Physical Properties of Alkanes
Chapter 1 of 3
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Chapter Content
Alkanes are almost non-polar molecules because of the covalent nature of C-C and C-H bonds and due to very little difference of electronegativity between carbon and hydrogen atoms. They possess weak van der Waals forces. Due to the weak forces, the first four members, C1 to C4 are gases, C5 to C17 are liquids and those containing 18 carbon atoms or more are solids at 298 K. They are colourless and odourless. What do you think about solubility of alkanes in water based upon non-polar nature of alkanes? Petrol is a mixture of hydrocarbons and is used as a fuel for automobiles. Petrol and lower fractions of petroleum are also used for dry cleaning of clothes to remove grease stains. On the basis of this observation, what do you think about the nature of the greasy substance? You are correct if you say that grease (mixture of higher alkanes) is non-polar and, hence, hydrophobic in nature. It is generally observed that in relation to solubility of substances in solvents, polar substances are soluble in polar solvents, whereas the non-polar ones in non-polar solvents i.e., like dissolves like.
Detailed Explanation
Alkanes are primarily composed of carbon and hydrogen atoms. The bonds between these atoms (C-C and C-H) are covalent, meaning they share electrons. Because the electronegativity difference between carbon and hydrogen is quite small, the overall molecule is nearly non-polar. As a result, alkanes do not dissolve well in water, which is a polar solvent. This explains why substances like petrol, which is made from hydrocarbons, are used for cleaning oily stains – oils and greases are also non-polar and mix well with alkanes. The physical state of alkanes changes with their molecular size: small alkanes (like methane) are gases, medium-sized ones (like pentane through heptadecane) are liquids, and larger ones (like eicosane) are solids. This observation about the physical state can be attributed to weak van der Waals forces between the molecules; as the molecule size increases, these forces become stronger, raising the boiling point and melting point.
Examples & Analogies
Think of alkanes like a set of Lego blocks. Smaller blocks might easily float on water because they don’t stick, just like how gases like methane diffuse easily into the air. If you start stacking more blocks together (making larger alkanes), they become heavier and denser. Eventually, if you stack enough blocks, they become solid and sink. Similarly, small alkanes evaporate easily, while larger alkanes don't dissolve in water, just as a big stack of blocks won’t float unless it’s designed to.
Boiling Points of Alkanes
Chapter 2 of 3
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Chapter Content
Boiling point (b.p.) of different alkanes are given in Table 9.2 from which it is clear that there is a steady increase in boiling point with increase in molecular mass. This is due to the fact that the intermolecular van der Waals forces increase with increase of the molecular size or the surface area of the molecule. You can make an interesting observation by having a look on the boiling points of three isomeric pentanes viz., (pentane, 2-methylbutane and 2,2-dimethylpropane). It is observed that pentane having a continuous chain of five carbon atoms has the highest boiling point (309.1K) whereas 2,2 – dimethylpropane boils at 282.5K. With increase in number of branched chains, the molecule attains the shape of a sphere. This results in smaller area of contact and therefore weak intermolecular forces between spherical molecules, which are overcome at relatively lower temperatures.
Detailed Explanation
The boiling point of alkanes tends to rise as the molecular mass increases. This rise is linked to the strength of the van der Waals forces, which are weak forces that act between molecules. When the size of an alkane increases, the surface area also increases, which allows these weak forces to become more significant, thereby elevating the boiling point. Consider the isomers of pentane: pentane (with a straight chain) has a higher boiling point than its branched isomers, 2-methylbutane and 2,2-dimethylpropane. Branching reduces the surface area available for these forces to act upon each other, resulting in weaker interactions and lower boiling points. Thus, in this case, a straight-chain molecule is more effective at packing closely together, leading to higher boiling points compared to branched versions.
Examples & Analogies
Imagine trying to stack boxes. If all boxes are lined up in a row (like the straight-chain pentane), you get a solid wall (high surface area), making it hard to get through—the heat needed to break this wall (boiling point) is high. However, if you rearrange some boxes into a cluster (like branched isomers), they take up less room, and you can squeeze through easily, needing less heat (lower boiling point) to break apart.
Chemical Properties of Alkanes
Chapter 3 of 3
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Chapter Content
As already mentioned, alkanes are generally inert towards acids, bases, oxidising and reducing agents. However, they undergo the following reactions under certain conditions: 1. Substitution reactions: One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group. Halogenation takes place either at higher temperature (573-773 K) or in the presence of diffused sunlight or ultraviolet light. Lower alkanes do not undergo nitration and sulphonation reactions. These reactions in which hydrogen atoms of alkanes are substituted are known as substitution reactions.
Detailed Explanation
Alkanes are typically considered non-reactive due to their stable C-H and C-C bonds. They are largely unresponsive to acids and bases, which is why they are classified as inert. However, they do participate in substitution reactions. This kind of reaction involves the replacement of hydrogen atoms in alkanes with other atoms or groups (like halogens or nitro groups). For example, during chlorination in the presence of UV light, a hydrogen atom can be replaced by a chlorine atom, forming alkyl halides. This can be likened to trading a toy (hydrogen atom) for a different toy (chlorine) without the entire structure falling apart; the alkane simply becomes something new while still keeping its core skeleton intact.
Examples & Analogies
Think of alkanes like a group of children playing in a park. They're all having fun together (the stable alkanes). Now, if one child (a hydrogen atom) trades places with a visiting child (a chlorine atom), the group still exists as a group of kids, but now with a new player. Just like in chemical reactions, the group can still function and play together despite having added someone new (chlorination).
Key Concepts
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Inertness of Alkanes: Alkanes are stable and do not react readily, but will undergo substitution reactions under specific conditions.
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Combustion Reaction: Alkanes combust with oxygen to produce carbon dioxide and water, releasing energy.
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Controlled Oxidation and Isomerization: Alkanes can be transformed into useful compounds through controlled reactions.
Examples & Applications
Combustion of Propane (C3H8): C3H8 + 5O2 → 3CO2 + 4H2O.
Hydrogenation of Butene: C4H8 + H2 → C4H10, using a catalyst.
Memory Aids
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Rhymes
Alkanes are stable, not quick to react, / Substitution for halogens is the fact.
Stories
Imagine an alkane sitting quietly at a party, not engaging until a halogen comes looking for a match, leading to a handshake, which represents substitution.
Memory Tools
To remember combustion: 'Cows Can Never Produce Heat' - Carbon dioxide, combustion, and products water.
Acronyms
Remember 'SIC' for alkanes
Stable
Inert
Combustion.
Flash Cards
Glossary
- Hydrocarbons
Compounds composed solely of hydrogen and carbon.
- Saturated
Refers to a hydrocarbon containing only single bonds between carbon atoms.
- Substitution Reaction
A reaction where one atom or group of atoms in a molecule is replaced by another atom or group.
- Combustion
A chemical reaction that typically involves the reaction of a fuel with oxygen to produce heat and light.
- Isomerization
The process where a molecule changes its structure to form isomers, which have the same molecular formula but different structural formulas.
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