L1-2 - Intro to Organic Chemistry

objectives

Understandings:

• A homologous series is a series of compounds of the same family, with the same general formula, which differ from each other by a common structural unit.
• Structural formulas can be represented in full and condensed format.
• Functional groups are the reactive parts of molecules.

Applications and Skills:

• Distinction between empirical, molecular and structural formulas.
• Identification of different classes:
  • alkanes
  • alkenes
  • alkynes
  • halogenoalkanes
  • alcohols
  • ethers
  • aldehydes
  • ketones
  • esters
  • carboxylic acids
  • amines
  • amides
  • nitriles
  • arenes
• Identification of typical functional groups in molecules eg:
  • phenyl
  • hydroxyl
  • carbonyl
  • carboxyl
  • carboxamide
  • aldehyde
  • ester
  • ether
  • amine
  • nitrile
  • alkyl
  • alkenyl
  • alkynyl
• Construction of 3-D models (real or virtual) of organic molecules.
• Application of IUPAC rules in the nomenclature of straight-chain and branched- chain isomers.

Guidance:

• Skeletal formulas should be discussed in the course
• non-cyclic alkanes and halogenoalkanes up to halohexanes.
• alkenes up to hexene and alkynes up to hexyne.
• compounds up to six carbon atoms (in the basic chain for nomenclature purposes) containing only one of the classes of functional groups:
  • alcohols
  • ethers
  • aldehydes
  • halogenoalkanes
  • ketones
  • esters
  • carboxylic acids.

What do we already know about organic chemistry?

• the chemistry of carbon compounds
• carbon has 4 electrons in its outer shell so it forms 4 covalent bonds
• these can be single bonds, double bonds or triple bonds
• carbon can form rings and long chains (which can be branched)

Why is Carbon so Special?

• CATENATION is the ability to form bonds between atoms of the same element.
• Carbon forms chains and rings, with single, double and triple covalent bonds, because it is able to FORM STRONG COVALENT BONDS WITH OTHER CARBON ATOMS
• Carbon forms a vast number of carbon compounds because of the strength of the C-C covalent bond. Other Group IV elements can do it but their chemistry is limited due to the weaker bond strength.

BOND ATOMIC RADIUS BOND ENTHALPY

C-C 0.077 nm +348 kJmol^-1

Si-Si 0.117 nm +176 kJmol^-1

• The larger the atoms, the weaker the bond. Shielding due to filled inner orbitals and greater distance from the nucleus means that the shared electron pair is held less strongly.

CHAINS AND RINGS

Carbon atoms can be arranged in:

You can also get a combination of rings and chains!

MULTIPLE BONDING AND SUBSTITUENTS

CARBON-CARBON COVALENT BONDS CAN BE SINGLE, DOUBLE OR TRIPLE

DIFFERENT ATOMS / GROUPS OF ATOMS CAN BE PLACED ON THE CARBONS

The basic atom is HYDROGEN but groups containing OXYGEN, NITROGEN, HALOGENS and SULPHUR are very common.

• The chemistry of an organic compound is determined by its FUNCTIONAL GROUP

ATOMS/GROUPS CAN BE PLACED IN DIFFERENT POSITIONS ON A CARBON SKELETON

THE C=C DOUBLE BOND IS IN A DIFFERENT POSITION

THE CHLORINE ATOM IS IN A DIFFERENT POSITION

Key Definitions

Empirical formula shows the ratio of atoms in a compound. It is the simplest ratio in which atoms combine to form a compound

Molecular formula tells us how many of each type of atom there are in each molecule of the compound

Full Structural formula shows all the bonds in the molecule as individual lines (each line represents a pair of shared electrons)

E.g. Methane

E.g. Propane

• Condensed Structural formula shows how the various atoms are bonded, without showing all the bonds - shows all atoms on each individual carbon

Methane CH₄

Propane CH₃CH₂CH₃

• Skeletal formula- all the hydrogen atoms are removed from carbon chains, leaving just a carbon skeleton with functional groups attached to it

E.g. Propane

E.g. Butane

E.g. Cyclohexane

E.g. Thalidomide

Homologous series and General Formula

Homologous series

• Family of organic compounds with the same functional group but different chain length.
• All compounds in a homologous series share the same general formula.
• Formula of a homologue differs from its neighbour by CH₂. (e.g. CH₄, C₂H₆, ... etc )
• Have similar chemical properties.
• Show a gradual change in physical properties as molar mass increases.
• Can usually be prepared by similar methods.

General formula

• a formula for a family of compounds. E.g. alkanes = C_nH_2n+2 where n is the number of carbon atoms

Methane: CH₄ n=1

C_nH_2n+2

C₁H_(2x1)+2

Propane: C₃H₈ n=3

C₃H_(2x3)+2

• General Formula do Not Work for Cyclic Hydrocarbons as they need two less Hydrogens

Functional Groups

• Organic chemistry is a vast subject so it is easier to split it into small sections for study. This is done by studying compounds which behave in a similar way because they have a particular atom, or group of atoms, FUNCTIONAL GROUP, in their structure.
• Functional groups can consist of one atom, a group of atoms or multiple bonds between carbon atoms.
• Each functional group has its own distinctive properties which means that the properties of a compound are governed by the functional group(s) in it.

Common Functional Groups

The IUPAC System of Nomenclature

• A system for naming compounds was developed by the International Union of Pure and Applied Chemistry (IUPAC)
• Systematic names tell us about the structure of a compound rather than just the formula

e.g. methylbutane vs C₅H₁₂

• There a series of IUPAC rules for naming compounds

IUPAC Rules

2) Prefixes and suffixes describe changes to the root molecule

a) Prefixes: added before a root

e.g. Side chains – name tells us the number of carbons

methyl- CH₃-

ethyl- C₂H₅-

propyl- C₃H₇- etc.

These are alkyl groups R in a general formula

e.g. Halogen substituent

Br- bromo- Cl- chloro etc.

b) Suffixes: added after a root

used to describes most functional groups (reactive groups of atoms)

3) To tell where a side chain or functional group is on the main chain we use the number of the carbon followed by a hyphen.

Give it the lowest possible number

4) If there is more than one side chain you place them in alphabetical order

e.g. 2-bromo-3-chlorohexane

Determining the Longest Chain

A simple way to check is to run a finger along the chain and see how many carbon atoms can be covered without reversing direction or taking the finger off the page. In all the above there are... FIVE CARBON ATOMS IN A LINE.

Naming Side Chains

SIDE-CHAIN

• carbon based substituents are named before the chain name.
• they have the prefix -yl added to the basic stem (e.g. CH₃ is methyl).

Rules for naming:

• Number the principal chain from one end to give the lowest numbers.
• Side-chain names appear in alphabetical order butyl, ethyl, methyl, propyl
• Each side-chain is given its own number.
• If identical side-chains appear more than once, prefix with di, tri, tetra, penta, hexa
• Numbers are separated from names by a HYPHEN e.g. 2-methylheptane
• Numbers are separated from numbers by a COMMA e.g. 2,3-dimethylbutane

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