Transcript Slide 1
ORGANIC REACTION Prepace What is Organic Chemistry? It is defined as the study of hydrocarbons (compounds of hydrogen and carbon) and their derivatives 7 million Organic Compounds 1.5 million Inorganic Compounds Animal and plant matter, Foods, Pharmaceuticals, Cosmetics, Fertilizers, Plastics, Petrochemicals, Clothing ORGANIC CHEMICALS ARE UNIVERSAL Biological matter •Plants •Animals •Microbes Geological matter •Fossil Fuels •Other Organic Chemicals Manufactured products Atmospheric and cosmic matter Nature give a lot of molecule Limited Quantity ORGANIC REACTIONS Rubber Limited Variations Making a compound (SYNTHESIS) Cosmetics . Dyes . . Drugs Limited Activity foods Organic Reaction Introduction Most molecules are at peace with themselves. Bottles of water, or acetone, or methyl iodide can be stored for years without any change in the chemical composition of the molecules inside. When we add chemical reagents, say, HCl to water, sodium cyanide (NaCN) to acetone, or sodium hydroxide to methyl iodide, chemical reactions occur. This chapter is an introduction to : 1. The reactivity of organic molecules (why they don’t and why they do react) 2. How we can understand reactivity in terms of charges and orbitals and the movement of electrons 3. How we can represent the detailed movement of electrons (the mechanism of the reaction) Molecules react because they move : • Whole molecules move continuously in space. • Bumping into each other, into the walls of the vessel they are in, and into the solvent if they are in solution. • When one bond in a single molecule stretches too much it may break and a chemical reaction occurs. • When two molecules bump into each other, they may combine with the formation of a new bond, and a chemical reaction occurs. Not all collisions between molecules lead to chemical change Organic molecules Many electrons at outer layer When will they react? Reaction will occur only if the molecules are given enough energy (the activation energy for the reaction) for the molecules to pass the repulsion and get close enough to each other All molecules repel each other Charge attraction brings molecules together Cations (+) and anions (–) attract each other electrostatically and this may be enough for reaction to occur. Na + + ClThis inorganic style of attraction is rare in organic reactions. A more common cause of organic reactions is attraction between a charged reagent (cation or anion) and an organic compound that has a dipole. It is not even necessary for the reagent to be charged. Ammonia also reacts with acetone and this time it is the lone pair of electrons. That is attracted to the positive end of the carbonyl dipole. Polarity can arise from σ bonds too. The most electronegative element in the periodic table is fluorine and three fluorine atoms on electropositive boron produce a partially positively charged boron atom by σ bond polarization. Summarize • The presence of a dipole in a molecule represents an imbalance in the distribution of the bonding electrons due to polarization of a σ bond or a π bond or to a pair of electrons or an empty orbital localized on one atom. • When two molecules with complementary dipoles collide and together have the required activation energy to ensure that the collision is sufficiently energetic to overcome the general electronic repulsion, chemical change or reaction can occur. Orbital overlap brings molecules together Other organic reactions take place between completely uncharged molecules with no dipole moments. The attraction between these molecules is not electrostatic. In fact, we know that reaction occurs because the bromine molecule has an empty orbital available to accept electrons. It is the antibonding orbital belonging to the Br–Br σ bond: the σ* orbital. There is therefore in this case an attractive interaction between a full orbital (the π bond) and an empty orbital (the σ* orbital of the Br–Br bond). The molecules are attracted to each other because this one interaction is between an empty and a full orbital and leads to bonding, unlike all the other repulsive interactions between filled orbitals. Molecules repel each other because of their outer coatings of electrons. Molecules attract each other because of : • attraction of opposite charges • overlap of high-energy filled orbitals with low-energy empty orbitals For reaction, molecules must approach each other so that • they have enough energy to overcome the repulsion • the right orientation to use any attraction ORGANIC REACTIONS CLASSIFICATIONS BASED ON MECHANISM REACTIONS : 1. ADDITION : - ELECTROPHILIC - NUCLEOPHILIC - FREE RADICAL 2. ELIMINATION : - E1 - E2 3. SUBSTITUTION : - NUCLEOPHILIC (SN1, SN2) - ELECTROPHILIC 4. REARRANGEMENT BASED ON OXIDATION NUMBER CHANGE : - ACID BASE - REDOX CLASSIFIED THIS REACTION Br 1. + CH2 Br-Br CH2-Br OH H2O + 2. 3. OH H2O + Cl O O 4. HCl + + H3C + H3C H2O CH3-NH2 OH NH-CH3 Br NO2 5. + O NO2 + Br2 OH HBr