Organic Chemistry Lecture 2 PDF

Electronegativity measures the ability of an atom to attract electrons it is a measure for reactivity. Electronegativity is affected by the number of protons and the number of shells holding the electrons. The higher number of protons the higher is the attraction for the electrons. 4- Hydrogen bon...

Electronegativity measures the ability of an atom to attract electrons it is a measure for reactivity. Electronegativity is affected by the number of protons and the number of shells holding the electrons. The higher number of protons the higher is the attraction for the electrons. 4- Hydrogen bonding A hydrogen bond is the electrostatic attraction between polar molecules that occurs when a hydrogen (H) atom bound to a highly electronegative atom such as nitrogen (N), oxygen (O) or fluorine (F) experiences attraction to some other nearby highly electronegative atom. The name hydrogen bond is something of a misnomer, as it is not a true bond but a particularly strong dipole-dipole attraction, and should not be confused with a covalent bond Hybrid orbitals are mixed orbitals—they result from combining orbitals. The concept of combining orbitals is called orbital hybridization. If the one s and three p orbitals of the second shell are combined and then apportioned into four equal orbitals, each of the four resulting orbitals will be one part s and three parts p. This type of mixed orbital is called an (stated “s-p-three” not “s-p-cubed”) orbital. The superscript 3 means that three p orbitals were mixed with one s orbital to form the hybrid orbitals. Each orbital has 25% s character and 75% p character. Types of chemical reactions 1- Substitution reaction Is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions are of prime importance in organic chemistry. Substitution reactions in organic chemistry are classified either as electrophilic or nucleophilic depending upon the reagent involved 2- Addition Reactions Is a reaction in which atoms or groups are attached to the molecule without displacement of any other atoms or groups. Only unsaturated compounds give addition reaction 3- Elimination Reactions Is a reaction in which atoms or groups are detached from the molecule without displacement of any other atoms or groups. This reaction is reverse of addition reaction Chemical formula in organic chemistry Lewis formulas are useful for keeping tack of bonding electrons, but organic chemists rarely use true Lewis formulas. Let us consider the types of chemical formulas that are more frequently encountered. 1- An empirical formula tells us the types of atoms and their numerical ratio in a molecule. For example, a molecule of ethane contains carbon and hydrogen atoms in a ratio of 1 : 3; the empirical formula is CH3. 2- A molecular formula tells us the actual number of each type of atom in a molecule, not the ratio. The molecular formula for ethane is C2H6. Chemical formula in organic chemistry 3- A structural formula shows the structure of a molecule, that is the order of attachment of the atoms. In order to explain or predict the chemical reactivity, we need to know the structure of a molecule. Therefore, structural formulas are the most useful of the different types of formulas. H H H C C H CH3 C2H6 H H empirical formula molecular formula structural formula (for ethane) Structural Formula REPRESENTATION OF STRUCTURAL FORMULA Acids and Bases In the Brønsted–Lowry definitions, an acid is a species that donates a proton, and a base is a species that accepts a proton. (Remember that positively charged hydrogen ions are also called protons.) Hydrogen chloride (HCl) meets the Brønsted–Lowry definition of an acid because it donates a proton to water Lewis theory of acids and bases: Lewis offered new definitions for the terms “acid” and “base.” He defined an acid as a species that accepts a share in an electron pair and a base as a species that donates a share in an electron pair. All proton-donating acids fit the Lewis definition because all proton-donating acids lose a proton and the proton accepts a share in an electron pair The Nature of Chemical Reactions the chemical reaction between two substances involves the breaking of already existing bond and formation of a new one. Energy in chemical reaction as energy is evolved when a bond is formed but when it breaks it needs the same amount of energy. The breaking of a simple covalent bond can take place in two different ways 1- Heterolysis (Heterolytic Fission): In this type of fission the bond is broken unsymmertically, i.e. the two electrons are retained by one atom. Such fissure results in the formation of ions. A: +B A : B A + :B 2- Homolysis (Homolytic Fission): This type involves the symmetrical breaking of the shared electron pair, one electron being retained by each atom. A:B A. + B. hemolytic or free radical reactions Heterolysis of carbon-carbon bonds. Carbocations & carbanions Depending on the electronegativity of the atom attached to carbon in comparing with carbon  Types of Reagents 1) Nucleophiles or nucleophilic reagents (electron donors): are nucleus loving reagents which in a reaction donate electrons to share by its with the center attached N: + R :X R: N + :X A- Negatively charged anions as OH-, CN-, RO- OH + R X R OH + X B- Neutral molecules having lone pair of electrons R3N: H3N: H2O: R2O: R2S : 2) Electrophiles or Electrophilic reagents (electron acceptors): electron loving reagents Positively charged cations Such as, H+, NO2+, X+, CH3+ Neutral molecules (electron deficient ) Such as FeCl3, AlCl3, BF3, C=O