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AS Introduction to organic chemistry

From your Syllabus 

  • 1)      Formulae, functional groups and the naming of organic compounds
    • a) interpret and use the general, structural, displayed and skeletal formulae of the following classes of compound:
      • i) alkanes, alkenes and arenes
      • ii) halogenoalkanes and halogenoarenes
      • iii) alcohols (including primary, secondary and tertiary) and phenols
      • iv) aldehydes and ketones
      • v) carboxylic acids, esters and acyl chlorides
      • vi) amines (primary only), nitriles, amides and amino acids

(Candidates will be expected to recognise the shape of the benzene ring when it is present in organic compounds. Knowledge of benzene or its compounds is not required for AS Level.)

    • b) understand and use systematic nomenclature of simple aliphatic organic molecules with functional groups, up to six carbon atoms (six plus six for esters and amides, straight chains only)
    • c) understand and use systematic nomenclature of simple aromatic molecules with one benzene ring and one or more simple substituents, for example 3-nitrobenzoic acid, 2,4,6-tribromophenol
    • d) deduce the possible isomers for an organic molecule of known molecular formula
    • e) deduce the molecular formula of a compound, given its structural, displayed or skeletal formula.
  • 2)      Characteristic organic reactions
    • a)       interpret and use the following terminology associated with organic reactions:
      • i)      functional group
      • ii)     homolytic and heterolytic fission
      • iii)    free radical, initiation, propagation, termination
      • iv)    nucleophile, electrophile
      • v)    addition, substitution, elimination, hydrolysis, condensation
      • vi)    oxidation and reduction (in equations for organic redox reactions, the symbols [O] and [H] are acceptable for oxidising and reducing agents)
  • 3)      Shapes of organic molecules; σ and π bonds
    • a)        
      • i)        describe and explain the shape of, and bond angles in, the ethane, ethene and benzene molecules in terms of σ and π bonds ii)       predict the shapes of, and bond angles in, other related molecules
  • 4)      Isomerism: structural and stereoisomerism
    • a) describe structural isomerism and its division into chain, positional and functional group isomerism
    • b) describe stereoisomerism and its division into geometrical (cis-trans) and optical isomerism (use of E, Z nomenclature is acceptable but is not required)
    • c) describe cis-trans isomerism in alkenes, and explain its origin in terms of restricted rotation due to the presence of π bonds
    • d) explain what is meant by a chiral centre and that such a centre normally gives rise to optical isomerism (Candidates should appreciate that compounds can contain more than one chiral centre, but knowledge of meso compounds, or nomenclature such as diastereoisomers is not required.)
    • e)  identify chiral centres and cis-trans isomerism in a molecule of given structural formula

Introduction to organic chemistry

Important facts

    • 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 Vaals' 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:

NAME

HEXANE DIMETHYLBUTANE

DISPLAYED FORMULA

n-hexane.png    2-2-dimethylbutane.png   

 DISPLAYED FORMULA

(all bonds are shown)

 STRUCTURAL FORMULA

(only important bonds are shown

2-2-dimethylbutane condensed

 SKELETAL FORMULA

(lines represent C-C bonds only)

skeletal-formula-2-2-dimethylbutane.png

 
MOLECULAR FORMULA

(No bonds are shown)

 C6H14 C6H14


EMPIRICAL FORMULA

(smallest ratio between atoms in a formula)

C3H7 C3H7

GENERAL FORMULA

1 Algebraic formula for all compounds
in a functional group or homologous series. 

CnH(2n+2) CnH(2n+2)
Property:  BOILING POINT/ °C 68 50

 

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 Vaal's forces. (Also called London Dispersion Forces)

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 hybridisation will be sp
  • If Carbon is attached to 3 elements, it will form 2 SINGLE and 1 DOUBLE covalent bonds and its hybridisation will be sp2
  • If Carbon is attached to 2 elements, it will form 1 SINGLE and 1 TRIPLE covalent bonds and the hybridisation 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.
  • Carbons attached through a triple bond, will have 1 SIGMA σ and a 2 PI π bonds

 For more details on sigma and pi bond, see the HYBRIDIZATION POWER POINT PRESENTATION

name formula hybridisation angle bond type
 methane  methane-wedge.png sp3 109.5° sigma
ethane ethane sp3 109.5° sigma

methanal

or

Formaldehyde

    formaldehyde.png      sp2 120°

1 sigma σ and  1 pi π

sigma-pi-bond-formaldehyde.png

Hydrogen Cyianide   hydrogen-cianide.png   sp 180°

1 sigma σ and  2 pi π  

sigma-pi-bond-hydrogen-cianide.png

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)

TABLE 1: PREFIXES 

ORGANIC CHEMISTRY PREFIXES

organic functional groups table

Homologous series

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:

Name  General Formula
Alkanes   CnH(2n+2)
Cycloalkanes CnH2n
Alkenes CnH2n
Alcohols CnH(2n+1)OH
Haloalkanes CnH(2n+1)X (X= F, Cl, Br, I)

Examples of the first 6 compounds in the alkanes homologous series are in the table below

Name  Molecular Formula Condensed Formula Difference with the compound above
methane    CH4 CH4 --
ethane C2H6 CH3CH3 CH2
propane C3H8 CH3CH2CH3 CH2
butane C4H10 CH3CH2CH2CH3 CH2
pentane C5H12 CH3CH2CH2CH2CH3 CH2
hexane C6H14 CH3CH2CH2CH2CH2CH3 CH2

Isomers:

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: Have different arrangements of atoms in their formula. There are three type of structural isomers:

  • Chain isomers: same formula and different arrangements.
n-hexane.png    2-2-dimethylbutane.png  

Positional Isomers: same functional in a different position.

  1-chloropropane.png     2-chloropropane.png   

 

  • Functional group isomers: same atoms but arranged in a different functional group 

 

  propanal.png       propanone.png 

     

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). 
    • Cis is also called Z (from zusammen, German: [tsuˈzamən], the German word for "together")
    • Trans is also called E (from entgegen, German: [ɛntˈɡeːɡən], the German word for "opposite").

 

 

 images/cis.png         
trans.png
   cis-2.png trans-2.png
  • Optical Isomers: they are isomers that rotate the plane of polarized light. 
    • They have 4 different substances attached to a single carbon atom. 
    • tow 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 racemises the "S" enantiomer producing the "R" enantiomer which caused serious malformation in babies. 

 

        optical-isomer.png        -           optical-isomer2.png  +   

       optical-isomer-thalidomide.png   

 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

Homolytic Fission

Some reactions occur ONLY in the presence of a free radical.
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.
 
 
This type of fission is called HOMOLYTIC. So, Chlorine molecules will split HOMOLITICALLY  to form CHLORINE RADICALS. 
This formation of the free radicals will trigger the chain reaction for the chlorination of methane (we will see this again in Alkanes). 
 
There are three steps in the reaction:I
  1. Initiation: the free radical forms thanks to the UV ENERGY.


  1. Propagation: the free radical reacts with more molecules forming more free radicals

  1. Termination: two free radicals get together forming new molecule


Heterolytic fission

The covalent bond is broken and the most electronegative element will take both electrons forming two ions
 

heterolytic-fission.png

Chloromethane bond is broken to form
a chloride ion(-) and a carbocation(+)

 

Nucleophiles

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. 
 
 
Electrophiles
 
 
    • v)    addition, substitution, elimination, hydrolysis, condensation
    • vi)    oxidation and reduction (in equations for organic redox reactions, the symbols [O] and [H] are acceptable for oxidising and reducing agents)

 

 


Extra information:

 

 

Presentation in Powerpoint  pptx      pdf

In your exam you should be able to recognize any of the following functional groups:

 

 

 

 

   
© Analia Sanchez