Intermolecular Forces

• Intermolecular forces are forces of attraction specifically when referring to liquids and gases

Intermolecular vs Intramolecular Forces

• Intermolecular forces are the forces seen between molecules
• Intramolecular forces are the forces seen within a molecule
• Intramolecular forces of attraction are much stronger than intermolecular forces

London Dispersion Force

• London Dispersion Force is present in all substances with electrons and is sometimes called induced dipole force.
• Fritz London described London dispersion force in the early 1900s
• London dispersion force is a temporary attractive force
• Temporary attractive force happens when electrons in two adjacent atoms occupy positions to form temporary dipoles
• Uneven distribution of electrons results in creation of temporary dipoles
• Weak intermolecular force results from the motion of electrons that creates temporary dipoles in molecules
• This interaction happens because of instantaneous position of an electron in a molecule, which temporarily makes that point of the molecule negatively charged, with the rest positively charged
• London dispersion forces are the only type of intermolecular force that operates between non-polar molecules
• The more polar the molecule, the higher its boiling point

Dipole-Dipole Force

• Dipole-dipole forces are attractive forces between polar molecules
• Dipole-dipole force is the attractive force between the positive end of one polar molecule and the negative end of another polar molecule
• Polar molecules have a partial negative end and a partial positive end
• The partially positive end of a polar molecule is attracted to the partially negative end of another
• Oppositely charged ends of polar molecules attract each other

Ion-Dipole Interactions

• Ion–dipole interactions are a stronger type of electrostatic interaction are important in solutions of ions
• The strength of these electrostatic forces makes it possible for ionic substances to dissolve in polar solvents

Hydrogen Bond Forces

• Hydrogen bonding occurs between polar covalent molecules that possess a hydrogen atom bonded to an extremely electronegative element (Nitrogen, Oxygen or Fluorine)
• Weak attractions occur between the hydrogen atoms of one molecule and the oxygen atom of another
• The dipole–dipole interactions experienced when Hydrogen is bonded to N, O, or F are strong
• Hydrogen bonds are the strongest of all of the intermolecular forces
• They are about one-tenth the strength of a covalent bond
• Substances that have significant hydrogen bonding will have higher than normal melting and boiling temperatures

Trends in Boiling Point

• Figuring out the order of boiling points is all about understanding trends, and is related to the strength of forces between molecules
• The more the molecules stick together, the more energy it will take to blast them into the atmosphere as gases
• There are 3 important trends to consider
• The relative strength of the four intermolecular forces is: Ionic > Hydrogen bonding > dipole dipole > Van der Waals dispersion (London Dispersion) forces
• The influence of each of these attractive forces will depend on the functional groups present
• Boiling points increase as the number of carbons is increased
• Branching decreases boiling point

Intermolecular Forces and Molecular Weight

• Van der Waals dispersion forces are proportional to surface area
• Increasing the length of the chain increases the surface area, which increases the ability of individual molecules to attract each other

Hydrocarbons

• Aliphatic carbons are arranged in chains
• Saturated hydrocarbons contain the maximum number of hydrogens
• Single bonds exist between all carbons (Alkanes)
• Unsaturated hydrocarbons contain 1+ double or triple bonds (Alkenes and Alkynes)
• Cyclic carbons are arranged in rings
• Aromatic carbons contain a benzene ring

Alkane

• Alkane is a hydrocarbon with only single bonds between carbon atoms, with the formula of CnH2n+2

Alkene

• Alkene is a hydrocarbon with at least one double bond and has the formula of CnH2n

Alkyne

• Alkyne is a hydrocarbon with at least one triple bond and has the formula of CnH2n-2

Alkyl Groups

• Alkyl Groups are fragment of an alkane molecule formed by the removal of one H atom which is represented by R
• Name ends in suffix – yl

Nomenclature of Branched Alkanes

• Count the number of Carbons in the longest chain
• Determine the appropriate root name
• Number the carbon atom starting from the end closer to the substituent
• Proceed the root name with substituent name

Important Rules for Naming Branched Alkanes

• Start numbering from the end that will give you the lowest number of branches
• If there is more than one type of branch, name the branches in alphabetical order
• If there is more than two of the same type of branch, give the branch a position number and prefixes “di”, “tri” “tetra” etc
• Put commas between numbers and hyphens between numbers and letters

Cyclic Alkanes and Substituents

• In the case of a cycloalkane with an attached alkyl substituent, the ring is considered the parent hydrocarbon
• There is no need to number the position of a single substituent on a ring
• If the ring has two different substituents, they are cited in alphabetical order and the number 1 position is given to the substituent cited first

Hydrocarbon Uses

• 1-4 C atoms are Gas and used for heating and cooking fuel
• 5-7 C atoms are liquids, (low boiling) and used for solvents, gasoline
• 6-18 C atoms are liquids, and used for gasoline
• 12-24 C atoms are liquids and used for jet fuel; camp stove fuel
• 18-50 C atoms are liquids, (high boiling) and used for diesel fuel, lubricants, heating oil
• 50+ C atoms are solids, and used for petroleum jelly, paraffin wax

Boiling and Melting Points of Alkanes

• The more Carbon atoms ↑ leads to boiling and melting points ↑
• The more branches ↑ leads to boiling and melting points↓

Chemical Reactions of Alkanes: Combustion

• Alkanes React with Oxygen
• The products of the reaction are CO2, H2O, and energy

Halogenation

• Alkanes react with Halogens

Nitration

• Heating an alkane in the vapor state with vapors of nitric acid at 420 degrees Celsius

Sulfonation

• Reaction with Sulfuric Acid

Addition Reaction

• Addition of HX (HCl, HBr, or HI) to an alkene gives a haloalkane
• Hydrogen adds to one carbon of the Carbon double bonded to Carbon (C=C) and X to the other
• If starting with a nonsymmetrical alkene, follow Markovnikoff's Rule and The negative group (Halogens) of the acid, attaches itself to the unsaturated carbon that has the fewest hydrogen.

Electronic Effects

• Electronic effects are the effects originating/present in organic molecules affecting part of a molecule’s reactivity via electron attraction/repulsion.

Inductive Effect

• Displacement of electrons along a saturated carbon chain whenever electron withdrawing. or electron donating group is present end of carbon chain
• Effect weakens steadily with increasing distance from the substituent

Two Types of Mesomeric or Resonance Effect

• • R or +M effect: if a group donates electrons to the conjugated-system through resonance, then it is said to have + R or + M effect
• – R or –M effect: Groups that withdraw electrons towards themselves through resonance are said to have – R or –M effect

Electromeric Effect

• Movement of electrons within a molecule containing double or triple bond
• It is a temporary effect operating in unsaturated compounds only at demand of reagents
• As soon as this attacking reagent is removed, the original condition is restored
• effect, the PI electrons flow to more electronegative/attacking reagent
• Involves complete transfer of PI electrons of multiple bonds

Steric Effect

• Amount of space that a group of atoms takes is called steric bulk
• effect is an influence on the shape of a molecule or on a reaction course or the relative rate of reaction caused by the space-filling properties.

Steric Hindrance

• Decrease in compound reactivity from bulky groups presence
• Occurs when spatial arrangement & large size of atoms or group hinder/retard chemical reaction.

Steric Shielding

• Group is so large that it protects nearby reactive groups from contact with substances that would normally react with them