Chapter 2 Reading Notes
Chapter 2 Reading Notes CHMY 321-001
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This 6 page Class Notes was uploaded by Rebeka Jones on Monday September 19, 2016. The Class Notes belongs to CHMY 321-001 at Montana State University taught by Holmgren, Steven in Fall 2016. Since its upload, it has received 3 views.
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Date Created: 09/19/16
Chapter 2 The advantage of Lewis structures is that they show all atoms and bond explicitly. But they are only useful for small molecules. Partially condensed structures – the C-H bonds are not all drawn explicitly – still only useful for small molecules Condensed structures – single bonds are not drawn. Instead groups of atoms are clustered together when possible. For example: if a molecule has two CH 3roups it would be written (CH ) . Thi3 2till is impractical for large molecules. *molecular formulas only show the number of atoms. It does not show structural information Molecular formulas do not provide enough information. Lewis structures take too long to draw, and partially condensed and condensed structures are only suitable for simple molecules. So we turn to bond line structures. Bond-line structures – simplify the drawing process and are easier to read. Most of the atoms are not drawn but with times these become user friendly -bond-line structures are drawn in a zig zag format and each corner or end point represents a carbon atom. -double bonds are shown with two lines and triple bonds are shown with three lines. *triple bonds are drawn in a linear fashion because triple bonds involve sp- hybridization which have linear geometry. -hydrogen atoms are not show because it is assumed each carbon has enough hydrogen atoms to achieve four bonds. How to draw bond line structures -carbon atoms in a straight line should be drawn in a zig zag format -when drawing double bonds, draw all bonds as far apart as possible - when drawing single bonds, the direction in which the bonds are drawn is irrelevant -all heteroatoms (atoms other than carbon and hydrogen) must be drawn and any hydrogen atoms attached to a heteroatom must be dr awn -never draw a carbon with more than 4 bonds When given a Lewis structure -delete hydrogen atoms, except for those attached to heteroatoms -draw in a zig zag format -delete carbon atoms with bond line drawings it is easier to identify functional groups functional group – a characteristic group of atoms/bonds that possess a predictable chemical behavior The chemistry of every organic compound is determined by the functional groups present in the compound. Therefore, the classification of the compounds is based off of the functional groups. 2 *alkenes – carbon carbon double bonds alcohols – compounds possessing OH groups a carbon atom will generally have four bonds only when it doesn’t have a formal charge -when there is a positive formal charge carbon with form 3 bonds - when there is a negative formal charge carbon will form 3 bonds and hold a lone pair *formal charge must always be shown in bond line structure; lone pairs can be left out A bond-line structure is only clear if it contains either all of the lone pairs or all of the formal charges. Since there is normally more lone pairs than charges it is convention of always drawing formal charges and leaving off lone pairs Patterns you will see for oxygen atoms -a negative charge corresponds with one bond and three lone pairs. -absence of charge corresponds with two bonds and two lone pairs -positive charge corresponds with three bonds and one lone pair Patterns you will see with nitrogen atoms -negative charge corresponds with two bonds and two lone pairs -absence of charge corresponds with three bonds and one lone pair -positive charge corresponds with four bonds and no lone pairs 3 wedges – represent a group coming out of the page dash – represents a group going behind the page Ficher projections – used for cyclic compounds The way to deal with in adequacy of bond line structures is called resonance. According to this we draw one more bond-line and mentally meld them together. These are called resonance structures. They show how the positive charge is spread over two locations *we separate them with a straight two headed arrow and place brackets around them This metal meld is called a resonance hybrid Delocalization – spreading of charge Delocalization of either a positive charge or a negative charge stabilized a molecule. This often is referred to as resonance stabilization Curved arrows are tools necessary to draw resonance structures properly. Every curve arrow has a tail and head. *They do not represent the movement of electrons The arrows treat electron AS IF they were moving even though they actually are not moving at all. The tail shows where the electrons are coming from and the head shows where they are going. 4 There are two rules to follow when drawing curved arrows for resonance structures. 1. Avoid breaking single bonds 2. Never exceed an octet for second row elements (octet rule) *second curved arrows can remove violations Structures are not complete without drawing formal charges. There are five patterns to drawing resonance structures. 1) An allylic lone pair – lone pair by pi bond a. Lone pair to create pi bond – two curved arrows 2) An allylic positive charge – positive charge in allylic position a. One arrow – tail of curved arrow is placed on lone pair and the head is placed to form pi bond between lone pair and positive charge 3) Lone pair adjacent to a positive charge a. One arrow – tail of curved arrow is placed on a lone pair and the head is placed to for pi bond between lone pair and positive charge 4) Pi bonds between two atoms of difference electronegativity a. Move pi bond up onto the electronegative atom to become a lone pair 5) Conjugate pi bonds enclosed in a ring a. Conjugate pi bonds are enclosed in a ring of alternating double and single bonds…we push all of the pi bond over by one position A compound could have three resonance structures; the three structures might not contribute equally to the overall resonance hybrid. One might be major significance while another is insignificant 5 1) Minimizes charges a. Structures with more than two charges should be avoided 2) Electronegative atoms such as N, O, and Cl, can bear a positive charge but only if they possess an octet *the most significant resonance structures are generally those in which all the atoms have an octet. 3) Avoid drawing a resonance structure in which two atoms bear opposite charges -they are usually insignificant a lone pair that is allylic to a pi bond is said to be delocalized. They occupy a p orbital rather than a hybridized orbital. A localized lone pair does not participate in resonance. Whenever an atoms possess both a pi bond and a lone pair, they will not both participate in resonance. Functional Groups 1) Alkyl Halide: has C to halogen bonded to C-H a. X = F, Cl, Br, or I 2) Alkene: C double bond C 3) Alkyne: C triple bond C 4) Alcohol: C bonded to OH 5) Ether: C bonded to O bonded to C 6) Thiol: C bonded to SH 7) Sulfide: C bonded to S bonded to C 8) Aromatic (Arene): cyclic array of single and double C bonds 6
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