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Alkanes, Alkenes and Conjugated Dienes

Solubility of alkanes, Boiling points of alkanes, Melting points of alkanes, Classification of alkenes, Uses of alkenes, Stability of conjugated dye.


Hydrocarbons that are saturated are alkanes. A saturated hydrocarbon is an alkane whose chemical formula has only one hydrogen atom and one carbon atom. A CnH2n+2 alkane has two hydrogen atoms. Alkanes, such as ethane, propane, and butane, are four types of compounds.
  • A non-polar compound is an alkane. Carbon and hydrogen have almost no difference in electronegativities, so they lack any kind of polarity.
  • A typical characteristic of alkanes is their lower melting and boiling points. These atoms are easily breakable since their Van Der Waals forces are weak.
  • Molecules become stronger as they become bigger. The boiling and melting points of more complex alkanes increase.
  • Solids, liquids, and gases are all possible states of matter when they are pure. The natural state of unbranched alkanes is that they are gases. Methane, for example, is an alkane. Solids are all alkanes larger than hexadecane.
  • A weak van der Waal force makes them completely insoluble in water.
  • In contrast, they dissolve in organic solids. Van der Waal forces of Alkane van der Waal are disrupted here and replaced by troops of more recent van der Waal.

Solubility of alkanes

  • In general, alkanes are non-polar molecules since carbon and hydrogen have very little difference in electronegativity, and the C-C and C-H bonds have covalent bonds.
  • Molecules containing polarity will be soluble in polar solvents, whereas those containing non-polarity will be soluble in non-polar solvents. Due to this, alkanes are hydrophobic, that is, water cannot dissolve them.
  • They can be dissolved in organic solvents after the van der Waals forces have been overcome.

Boiling points of alkanes

  • In a molecule as its surface area or molecular size increases, the Van Der Waals forces increase.
  • The boiling point of alkanes increases with increasing molecular weight.
  • Comparing straight-chain alkanes to their structural isomers, straight-chain alkanes exhibit a higher boiling point.

Melting points of alkanes

  • The melting points of alkanes increase along with their boiling points with increasing molecule weight.
  • Higher alkanes are hard to dissolve in water due to intermolecular forces of attraction.
  • Alkanes with even numbers have a higher melting point than alkanes with odd numbers since, in the solid phase, even-numbered alkanes form an organized structure, making it difficult to break.


The chemical industry is extremely dependent on alkenes, especially ethene. Crude oil does not contain large amounts of these compounds, but they are created by cracking alkanes. Carbon dioxide and water are formed when alkenes burn in the air, just like hydrocarbons. The explosive reaction of ethene with oxygen prevents it from being used much as a fuel. Besides being valuable in the chemical industry for making plastic and other chemicals, alkenes are not suitable for use as fuels.

General properties of alkenes

Physical state - In the absence of oxygen, gases with two or four carbon atoms, liquids containing 18 and upwards, and solids at room temperature emit a luminous, smoky flame in the absence of oxygen.

Density - The density of alkenes is lower than that of water.

Solubility – Water is insoluble in alkenes, but they dissolve in organic solvents like benzene.

Boiling point - With an increase in molecular mass or chain length, the boiling points of alkenes increase gradually. This indicates the intermolecular attraction becomes stronger as the molecule gets bigger.

Classification of alkenes

The sp2 hybridized carbon atoms in alkenes bonded to alkyl groups affect the stability of double bonds. A alkene's chemical reactivity is typically determined by the number of alkyl groups attached to its sp2 hybridized carbon atoms. In alkenes, distinguishing them can be determined by the number of alkyl groups attached to the structural unit C=C. Such groups are referred to as degrees of substitution. In an alkene with a monosubstituted carbon atom, the sp2 hybridized carbon atom is attached to one alkyl group. Alkenes with double bonds at the end of their carbon chain are also called terminal alkenes. Those in which two, three, or four alkyl groups are substituted with a double bond are called disubstituted, trisubstituted, and tetrasubstituted alkenes, respectively.

Uses of alkenes

  • There are many uses for alkenes such as ethene and propene.
  • Plastics such as polythene can be used to make buckets, bags, and bowls.
  • Production of polystyrene used in the manufacture of refrigerator parts and car battery cases.
  • Antifreeze, ethane-1,2-diol is an ingredient in automotive radiators.
  • Synthetic fibers for both terylene and ethanol are being developed.
  • Creation of an anti-knock material for cars.
  • Ropes and packaging materials are made from plastic and polypropene.
  • Production of propanol for use in acetone production.
  • Acrylate fiber manufacture.

Conjugated dienes

Dienes are hydrocarbon chains that contain two double bonds that are either adjacent or not. Using two neighboring double bonds as an example, we explore the delocalization of pi systems. Depending on how these double bonds are arranged, a compound's reactivity and stability can vary.

Dienes with two double bonds connected by a single bond are called conjugated dienes.

Stability of conjugated dienes

Charge delocalization is promoted by factors such as resonance and hybridization energy in conjugated dienes (both isolated and accumulated). Allylic radicals possess higher stability than secondary and tertiary carbohydrate radicals. During the formation of the double bond, the pi orbitals overlap creating a strong single bond. Here's a resonance structure showing how the charge is distributed over the four carbon atoms in this conjugated diene. This delocalization of charges stabilizes the conjugated diene:
A compound's stability is also affected by the hybridization energy along with resonance. In 1,3-butadiene, the carbons are distinct and sp2 hybridized, whereas, in dienes with a single bond, the carbons have sp3 hybridization. Dienes with s-character exhibit greater pi-electron attraction, resulting in stronger and shorter bonds than C-C bonds in ordinary alkanes (1.54Å).

The energies of hydrogenation associated with different arrangements of double bonds are also important to consider. A conjugated diene has a lower heat of hydrogenation than this compound's isolated (~ 60 kcal) and cumulated (~ 70 kcal) counterparts since the heat of hydrogenation increases as the compound becomes more volatile. The following figure illustrates the relative stability of different molecules by comparing the heats of hydrogenation of different types of bonds:

Different conformations of conjugated dienes

Conjugated dienes can be expressed in s-cis or s-trans conformations. Double bonds in a molecule are cis in relation to a single bond while double bonds in a molecule are trans in relation to a single bond. Because hydrogens are sterically interdependent with carbon, the cis conformation is less stable. A cycloaddition reaction involving Diels-alder is one important application of a conjugated diene in its cis conformation. Even though the trans conformation is considered more stable, the molecule is still used in its cis conformation because it can interconvert and rotate around a single bond.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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