BP/MP/SOLUBILITY

  • Boiling Point (BP): The temperature at which a substance transitions from a liquid to a gas.

  • Melting Point (MP): The temperature at which a substance transitions from a solid to a liquid.

  • Solubility: The ability of a substance to dissolve in a solvent, typically expressed in grams of solute per liter of solution (g/L).


ALKANES: ( CnH2n+2CnH2n+2 )

  • Alkanes are hydrocarbons that contain only single or double bonds. They are called saturated because their carbon skeleton contains the maximum possible number of hydrogen atoms.

  • LDF occur because electrons move constantly, creating temporary uneven distributions of electron density called temporary dipoles.

  • Physical properties are governed by the intermolecular forces.

  • Straight chain alkanes only exhibit LDF: These attractions get stronger as the molecules get longer and have more electrons. This increases the sizes of the temporary dipoles that are permitted. This is why the boiling points increase as the number of carbon atoms in the chains increases.

  • Large non-polar hydrocarbon regions can dominate a molecule’s overall characteristics (the large non-polar part has greater influence.)


  1. Solubilility: is low or not at all in polar solvents due to non-polar characteristics of C-H bonds.

2. Melting and Boiling Points:

Shorter chain= less LDF= lower MP/BP

Longer chain= more LDF= higher MP/BP


Complex Branches/alkyl/substituents:


Alkyl Halides: halogen atom bonded to an alkyl (hydrocarbon) group

  • The location of the halogen often results in a polar compound. As a result, halogenalkanes tend to be more soluble than alkanes in polar solvents, and tend to have higher melting points compared to alkanes.


Alkenes: ( CnH2n)

  • Alkenes are hydrocarbons with a Carbon backbone joined by one or more double bonds. They are called “unsaturated”. The double bond is more reactive than single bonds, due to presence of the pi bond.

  1. Solubility:

  • Alkenes are generally non-polar, so they are insoluble in water.

  • Alkenes may become more soluble in water if substituted with polar functional groups, depending on the group.

  • As chain length increases, solubility in water decreases because the non-polar hydrocarbon portion becomes larger.

  • Non-polar substances generally dissolve in non-polar solvents (“like dissolves like”).

  1. Melting and Boiling Points:

  • Intermolecular forces: Alkenes experience London dispersion forces (LDF).

  • Polarity: Some alkenes may be polar depending on their structure and substituents, which can result in dipole–dipole interactions.

  • Chain length: As chain length increases, the number of electrons increases, strengthening LDF.

  • Boiling point (BP): Stronger LDF requires more energy to overcome, so BP generally increases as chain length increases.

  • Melting point (MP): Melting point depends on intermolecular forces and how well molecules pack together, so it does not always increase consistently with chain length.


Alkynes: ( CnH2n-2)

Alkynes are hydrocarbons with carbon-carbon triple bonds.

  • Ethyne: The most common alkyne. It is used as a fuel for welding torches because it burns with an extremely hot flame, which is useful for many industrial applications.

  • Reactivity of Alkynes: Alkynes are highly reactive because they contain two pi bonds.

  • Chemical Building Blocks: Because they react easily, alkynes are important chemical building blocks in synthetic chemistry and pharmaceutical chemistry.


  1. Solubility:

  • While alkenes can be technically polar, the non-polar characteristics of the C-H bonds and the overall structure tend to govern alkene solubility.

  • Alkynes are insoluble in water unless substituted with halides or the functional groups.

  • Solubility further decreases as chain length increases.


  1. Melting and Boiling Points:

  • Non-substituted alkynes

    • May be highly non-polar.

    • Their rigid, linear structure allows molecules to pack closely together.

    • Better molecular packing can result in slightly higher BP compared to alkenes.

    2. London Dispersion Forces (LDF)

    • The non-polar carbon chains experience LDF.

    • Longer carbon chains have more electrons, resulting in stronger LDF.

    • Stronger LDF requires more energy to overcome, so BP increases as chain length increases.

memorize

  • Longer chain → more electrons → stronger LDF → higher BP.

  • Rigid, linear alkynes → better molecular packing → potentially slightly higher BP than comparable alkenes.


2.2- AROMATIC HYDROCARBONS

Benzene's Stability

  • Double bonds usually undergo reactions readily.

  • If benzene behaved like a molecule containing three separate double bonds, we would expect a much greater enthalpy change during reaction.

  • Experimentally, benzene releases or absorbs less energy than expected for three separate double bonds.

  • This indicates that benzene has extra stability due to the delocalization of its π electrons.

Why is benzene more stable?

  • Its π electrons are spread over the entire ring instead of being confined to three separate double bonds. This delocalization lowers the molecule's energy and makes benzene unusually stable.



ALCOHOLS (R-O-H)

The physical and chemical properties of alcohols are mainly due to the presence of hydroxyl(OH) group and the ability to hydrogen bond (strong IMF).

  1. Boiling Point: Alcohols have generally high boiling point compared to HC of the same length/molar mass, due to strong IMF (hydrogen bonding between hydroxyl groups)

  • As chain length increases = BP increases

  • As branching increases(of aliphatic side chains) = BP decreases, due to decrease in surface area that’s in contact with other molecules.

add the Oh one here


  1. Solubility: H-bonding can occur between polar solvent (water) and alcohol molecules. BUT the alkyl group is hydrophobic/non-polar.

  • As chain length increases= solubility decreases, due to increase in surface area thats in contact with other molecules. this increases LDF.


Ethers (R-O-R’)

1. Boiling Point (BP) and Melting Point (MP)

  • Intermolecular forces: Ethers experience London dispersion forces (LDF) and dipole–dipole (DD) interactions.

  • No hydrogen bonding between ether molecules: Ethers do not have an O–H bond, so they cannot hydrogen-bond with each other.

  • Comparison with alkanes: Ethers generally have higher BP than comparable alkanes because ethers have dipole–dipole interactions in addition to LDF.

  • Comparison with alcohols: Ethers generally have lower BP than comparable alcohols because alcohols can form hydrogen bonds with each other.

  • Solubility:

  • Ethers can dissolve many polar and non-polar substances, so they are useful as solvents.

  • The oxygen atom in an ether has lone pairs and can accept hydrogen bonds from water.

  • Ethers cannot donate hydrogen bonds because they do not have an O–H bond.

  • Smaller ethers can be soluble in water, but solubility generally decreases as the carbon chain gets longer.


Aldehydes and Ketones — Physical Properties

Carbonyl group: a carbon atom double-bonded to an oxygen atom.

1. Boiling Points (BP)

  • London Dispercion Forces: These attractions get stronger as the molecules get longer and have more electrons. This increases the sizes of the temporary dipoles that are permitted. This is why the boiling points increase as the number why the boiling points increase as the number of carbon atoms in the chains increases.

  • Dipole Dipole Forces:

  • Aldehydes and ketones are polar because they contain a polar C=O bond.

  • Oxygen attracts the shared electrons more strongly than carbon, making oxygen partially negative (\(\delta^-\)) and carbon partially positive (\(\delta^+\)).

  • These permanent dipoles attract the opposite partial charges of nearby molecules, creating dipole–dipole forces.

  • Aldehydes and ketones experience both LDF and DD forces.

  • Therefore, they generally have higher BPs than similarly sized hydrocarbons, which experience only LDF.

  1. Solubility:

  • The C=O bond is polar, and its oxygen can accept hydrogen bonds from water molecules.

  • This allows small aldehydes and ketones to be highly soluble in water.

  • As the carbon chain gets longer, the non-polar hydrocarbon portion becomes larger.

  • Eventually, the non-polar portion has a greater influence on the molecule's overall behaviour in water, so water solubility decreases as chain length increases.


Carboxylic Acids — Physical Properties

1. Boiling Points (BP):

  • High boiling points: Carboxylic acids have high BP compared with other substances of comparable molar mass because of strong intermolecular forces.

  • Hydrogen bonding: The O–H group readily forms hydrogen bonds with other carboxylic acid molecules.

  • Three intermolecular forces: Carboxylic acids experience London dispersion forces (LDF), dipole–dipole (DD) forces, and hydrogen bonding (HB).

  • Effect of chain length: BP generally increases as molar mass and carbon chain length increase because larger molecules have more electrons, leading to stronger LDF.2. Solubility in Water

  1. Solubility in Water:

  • Hydrogen bonding with water: The O–H group and carbonyl oxygen allow carboxylic acids to form hydrogen bonds with water molecules.

  • Short carbon chains: Carboxylic acids containing 1–4 carbon atoms are very soluble in water.

  • Longer carbon chains: Water solubility decreases as chain length increases because the non-polar hydrocarbon portion becomes larger relative to the polar carboxyl group.

  • Example: Hexanoic acid (6 carbon atoms) has low water solubility compared with carboxylic acids containing 1–4 carbons.


Ester:

An organic compound characterized by the presence of a carbonyl group bonded to an oxygen atom.

Boiling Points:

● Esters are polar but does not have hydrogen bonding abilities

● Tend to be lower boiling point than carboxylic acids and amides

● Borderline solubility in molecules of 3-5 carbon atoms

Solubility:

● The ester group is polar, readily dissolves in water

● Solubility decrease as carbon chain length increase as LDF become more predominant (more non-polar characteristics)


Nitrogen In Organic Chemistry

  • Carbon has a maximum bonding capacity of 4. The most simple carbon molecule, is methane.

  • Nitrogen can exist similarly to that of carbon. It can replace a carbon in a chain/ring and has a maximum bonding capability of 3.

Melting Point

● capable of hydrogen bonding

● typically lower boiling point than alcohols. the OH bond in alcohol is a greater dipole

● IMF strongest in primary amines, compared to secondary and tertiary

● Tertiary amines have no H, so no ___H-bonding________

● Boiling point order: __________1>2>3_________________

● lower aliphatic amines are gas, with a fishy smell

● primary amines with 3-4 C are liquids, higher ones are solids

Solubility

● capable of hydrogen bonding

● shortered chained amines can form hydrogen bonds with water molecules, making them soluble in

water

● longer chain, larger molar mass, decreased solubility in water

● very long chain amines are insoluble in water

Amides

• An organic compound characterized by the presence of a carbonyl group bonded to a nitrogen atom

Melting Point:

● with the exception of formaide (HCONH2), which is a liquid, all simple amides are solids

● boiling point of amides are much higher than alcohols of similar mass

● generally high BP and MP due to the polar nature of the amide group, allowing for H bonding and dipole dipole interactions

Solubility:

capable of hydrogen bonding

● amides with 5 or less C are soluble in water

● shortered chained amines can form hydrogen bonds with water molecules, making them soluble in water

● longer chain, larger molar mass, decreased solubility in water

● very long chain amines are insoluble in water