Elfi Susanti VH SBI CLASS Chemistry Department FKIP UNS
Advices for studying organic chemistry <ul><li>Keep up with your studying day to day –– never let yourself get behind, or better yet, be a little ahead of your instructor. Organic chemistry is a course in which one idea almost always builds on another that has gone before. </li></ul><ul><li>Study materials in small units, and be sure that you understand each new section before you go on to the next. </li></ul><ul><li>Work all of the in-chapter and assigned problems. </li></ul><ul><li>Write when you study. Write the reactions, mechanisms, structures, and so on, over and over again. </li></ul>
<ul><li>Learning by teaching and explaining. Study with your student peers and practice explaining concepts and mechanisms to each other. </li></ul><ul><li>Use the introductory material in the Study Guide entitled “Solving the puzzle ––or –– Structure is everything (Almost)” as a bridge from general chemistry to your beginning study of organic chemistry. Once you have a firm understanding of structure , the puzzle of organic chemistry can become one of very manageable size and comprehensible pieces. </li></ul><ul><li>Use molecular models when you study. </li></ul>
To determine stability of compound as ion or neutral . . NH 2 - ( Formal Charge = 5 - 4 - 2 = -1 ) 5e - N : . .. N : . . . . H H
azide anion has two negative-charged nitrogens and one positive-charged nitrogen, the total charge being minus one. Ozone, the central oxygen atom has three bonds and a full positive charge while the right hand oxygen has a single bond and is negatively charged. The overall charge of the ozone molecule is therefore zero. nitromethane has a positive-charged nitrogen and a negative-charged oxygen, the total molecular charge again being zero.
_ +2 2 - Determine formal charges !!! S O O O O .. .. : .. : : .. .. : .. : : _ _ _ net ionic charge
Group Formula Class Name Specific Example IUPAC Name Common Name Alkene H 2 C=CH 2 Eth ene Ethylene Alkyne HC ≡ CH Eth yne Acetylene Arene C 6 H 6 Benzene Benzene
Group Formula Class Name Specific Example IUPAC Name Common Name Halide H 3 C-I Iodomethane Methyl iodide Alcohol CH 3 CH 2 OH Ethan ol Ethyl alcohol Ether CH 3 CH 2 OCH 2 CH 3 Diethyl ether Ether Amine H 3 C-NH 2 Aminomethane Methylamine Nitro Compound H 3 C-NO 2 Nitromethane Thiol H 3 C-SH Methane thiol Methyl mercaptan Sulfide H 3 C-S-CH 3 Dimethyl sulfide
Group Formula Class Name Specific Example IUPAC Name Common Name Nitrile H 3 C-CN Ethanenitrile Acetonitrile Aldehyde H 3 CCHO Ethan al Acetaldehyde Ketone H 3 CCOCH 3 Propan one Acetone Carboxylic Acid H 3 CCO 2 H Ethan oic Acid Acetic acid Ester H 3 CCO 2 CH 2 CH 3 Ethyl ethan oate Ethyl acetate Acid Halide H 3 CCOCl Ethan oyl chloride Acetyl chloride Amide H 3 CCON(CH 3 ) 2 N,N-Dimethylethanamide N,N-Dimethylacetamide Acid Anhydride (H 3 CCO) 2 O Ethan oic anhydride Acetic anhydride
Resonance This averaging of electron distribution over two or more hypothetical contributing structures ( canonical forms ) to produce a hybrid electronic structure
<ul><li>evaluating the contribution each of these canonical structures makes to the actual molecule: </li></ul><ul><li>The number of covalent bonds in a structure . </li></ul><ul><li>(The greater the bonding, the more important and stable the contributing structure.) </li></ul><ul><li>2. Formal charge separation . </li></ul><ul><li>(Other factors aside, charge separation decreases the stability and importance of the contributing structure.) </li></ul><ul><li>3. Electronegativity of charge bearing atoms and charge density . </li></ul><ul><li>(High charge density is destabilizing. Positive charge is best accommodated on atoms of low electronegativity, and negative charge on high electronegative atoms.) </li></ul>The stability of a resonance hybrid is always greater than the stability of any canonical contributor
structure of methane (CH 4 ), the 2s and three 2p orbitals must be converted to four equivalent hybrid atomic orbitals , each having 25% s and 75% p character, and designated sp 3 . These hybrid orbitals have a specific orientation, and the four are naturally oriented in a tetrahedral fashion.
Molecular Orbitals In general, this mixing of n atomic orbitals always generates n molecular orbitals.
Intermolecular Forces <ul><li>van der Waals attraction </li></ul><ul><li>Hydrogen Bonding </li></ul>
This attractive force has its origin in the electrostatic attraction of the electrons of one molecule or atom for the nuclei of another. If there were no van der Waals forces, all matter would exist in a gaseous state , and life as we know it would not be possible. It should be noted that there are also smaller repulsive forces between molecules that increase rapidly at very small intermolecular distances.
<ul><li>some of the factors that influence the strength of intermolecular attractions: </li></ul><ul><li>The formula of each entry is followed by its formula weight in parentheses and the boiling point in degrees Celsius. </li></ul><ul><li>Molecular size. </li></ul><ul><li>Large molecules have more electrons and nuclei that create van der Waals attractive forces, so their compounds usually have higher boiling points than similar compounds made up of smaller molecules. </li></ul><ul><li>Molecular shape </li></ul><ul><li>Molecular dipoles generated by polar covalent bonds </li></ul>
Hydrogen Bonding <ul><li>The most powerful intermolecular force influencing neutral (uncharged) molecules </li></ul><ul><li>Hydrogen forms polar covalent bonds to more electronegative atoms such as oxygen, and because a hydrogen atom is quite small, the positive end of the bond dipole (the hydrogen) can approach neighboring nucleophilic or basic sites more closely than can other polar bonds. </li></ul>
<ul><li>The molecule providing a polar hydrogen for a hydrogen bond is called a donor . </li></ul><ul><li>The molecule that provides the electron rich site to which the hydrogen is attracted is called an acceptor . </li></ul><ul><li>Water and alcohols may serve as both donors and acceptors, whereas ethers, aldehydes, ketones and esters can function only as acceptors. </li></ul><ul><li>Primary and secondary amines are both donors and acceptors, but tertiary amines function only as acceptors. </li></ul>
All atoms and molecules have a weak attraction for one another, known as van der Waals attraction . This attractive force has its origin in the electrostatic attraction of the electrons of one molecule or atom for the nuclei of another, and has been called London dispersion force . In general, larger molecules have higher boiling points than smaller molecules of the same kind, indicating that dispersion forces increase with mass, number of electrons, number of atoms or some combination thereof. The following table lists the boiling points of an assortment of elements and covalent compounds composed of molecules lacking a permanent dipole. The number of electrons in each species is noted in the first column, and the mass of each is given as a superscript number preceding the formula.