AS Introduction to organic chemistry
- Organic Chemistry is also known as the chemistry of Carbon.
- Carbon atoms form covalent bonds with itself forming chains that lead to a huge amount of compounds.
- Besides carbon atoms, organic compounds often also contain Hydrogen, Nitrogen and Oxygen among other elements.
- Organic compounds present different arrangement of atoms with the same molecular formula. These compounds are called isomers.
Formulae and Structure
Knowing the structure of an organic compound is very important since physical and chemical properties depend basically on three factors:
- The number of carbon atoms present in the formula
- The higher the number of carbon atoms, the higher the melting point, boiling point and the viscosity.
- The arrangement of the atoms present.
- Straight chain arrangement facilitates the presence of stronger Van der Waals’ forces, comparing them to the branched compounds. This is one of the reasons why straight chain molecules have higher melting and boiling point and viscosity
- The functional group present in the formula.
- Functional groups are atoms or groups of atoms which will give the molecule certain physical and chemical properties.
For all the reasons above, the molecular formula gives us only limited information about the chemical and properties of the compound. That’s why we need to use different ways to represent organic compounds.
Depending on the substance and the property to be analyzed, we will use one of these formula formats:
In the table above, you can see two compounds with same molecular formula have different boiling points due to their different arrangement of atoms. these are two isotopes of the compound C6H14.
The molecules with the same number of atoms and straight chains, present stronger intermolecular forces comparing them with the branched isomers.
Note: depending on the structure and functional group present, the intermolecular forces may vary.
Hydrocarbons present only Van der Waals forces. (Also called London Dispersion Forces)
Alcohols present Van der Waals forces and hydrogen bonds, because of the hydroxyl group (-OH) attached to the alkyl group (Group of carbons and Hydrogens only)
Bonding in organic molecules
Atoms in organic compounds are bonded mainly through covalent bonds.
Depending on the number of atoms bonded to the carbon atom, the hybridisation of the carbon atom may vary. See table below.
- If Carbon is attached to 4 elements, it will form 4 SINGLE covalent bonds and its hybridization will be sp3
- If Carbon is attached to 3 elements, it will form 2 SINGLE and 1 DOUBLE covalent bonds and its hybridization will be sp2
- If Carbon is attached to 2 elements, it will form 1 SINGLE and 1 TRIPLE covalent bonds and the hybridization will be sp1
- Single bonds between atoms are always SIGMA σ. In this type of bond, the atoms orbitals approach each other frontally.
- Carbon atoms attached through double bond, will have 1 SIGMA σ and a 1 PI π bonds between them.
- π bonds form when two p orbitals approach sideways.
- In the picture below is shown the double bond.
- Carbons attached through a triple bond, will have 1 SIGMA σ and a 2 PI π bonds
Naming Organic Compounds
The names of simple organic compounds are composed of two parts –
- A prefix which comes from the number of carbon atoms in the longest straight chain in the molecule
(See table 1 = PREFIXES)
- 2) A suffix which shows the functional group that specifies which type of different organic compound it is
(See table 2: ORGANIC FUNCTIONAL GROUPS)
Functional Groups Table
For your exam, you do not need to know all functional groups
- AS level required functional groups are marked with a GREEN rectangles
- A2 level required functional groups are marked in VIOLET – besides the GREEN ones
They are groups of organic compounds that have the same functional group and different amount of carbons in the carbon chain.
All compounds in the same homologous series have the same general formula.
General formulas for some homologous series are:
Examples of the first 6 compounds in the alkanes homologous series are in the table below
Isomers are compounds which have the same molecular formula and different displayed formula. That means, the number and kind of atoms are the same but they are arranged differently.
There are two main kind of isomers:
- STRUCTURAL ISOMERS
- Chain Isomers
- Positional Isomers
- Functional Group Isomers
- Cis-Trans Isomers
- Optical Isomers
STRUCTURAL ISOMERS: Have different arrangements of atoms in their formula. There are three type of structural isomers:
- Chain isomers: same formula and different arrange
- Left: Hexane
- Right: 2,2-dimethylbutane
- Positional Isomers: same functional in a different position.
- Left: 1-chloropropane
- Right: 2-chloropropane
- Functional group isomers: same atoms but arranged in a different functional group
- Left: propanal
- Right: propanone
STEREOISOMERS: Same atoms bonded to each other but with a different arrangement into space. There are two types of stereoisomers:
- Cis-Trans Isomers: Double bond cannot rotate, Unsaturated organic compounds form two type of isomers= Cis (E) and Trans (Z).
OPTICAL ISOMERS: they are isomers that rotate the plane of polarized light.
They possess the following characteristics:
- They have 4 different substances attached to a single carbon atom.
- The carbon that is attached to four different substances is called a ‘CHIRAL CENTER” or CHIRAL CARBON.
- Two optical isomers are a mirror reflection of each other.
- Both optical isomers are called enantiomers.
- They have identical chemical and physical properties but they affect the polarized light and they react different with other chiral molecules.
- A mixture of 50%/50% of each optical isomer will not rotate light. this is called a racemic mixture.
- In 1960’s thalidomide was prescribed to pregnant women to help with morning sickness. the body racemizes the “S” enantiomer producing the “R” enantiomer which caused serious malformation in babies.
If the center is oriented so that the lowest-priority of the four is pointed away from a viewer, the viewer will then see two possibilities: If the priority of the remaining three substituents decreases in clockwise direction, it is labeled R (for Rectus, Latin for right), if it decreases in counterclockwise direction, it is S (for Sinister, Latin for left).
ORGANIC REACTION AND MECHANISMS
A chemical reaction can be explained following a series of steps called mechanisms. All mechanisms use at least one of the following features
Some reactions occur ONLY in the presence of a free radical.
A free radical is an atom with an unpaired electron that once belonged to a bond.
Halogens form free radicals in presence of UV light.
NOTE THE HALF ARROW, INDICATING THAT ONLY ONE ELECTRON IS BEING TRANSFERRED TO EACH CHLORINE.
- Initiation: the free radical forms thanks to the UV ENERGY.
2. Propagation: the free radical reacts with more molecules forming more free radicals
3. Termination: two free radicals get together forming new molecule
The covalent bond is broken and the most electronegative element will take both electrons forming two ions
Chloromethane bond is broken to form a chloride ion(-), a nucleophile, and a carbocation(+) an electrophile.
These are species with lone pairs of electrons, most of the time, negative ions which will be attracted to positive ions or carbocations and they are electron pair donors.
In the picture above, the Chloride ion is a NUCLEOPHILE.
Examples of nucleophiles are the halogen anions (I-, Cl-, Br-), the hydroxide ion (OH-), the cyanide ion (CN-), ammonia (NH3), and water.
These are species that Accept a pair of electrons to form a new covalent bond