REVISED 10/14 EMISTRY 1101L MOLEULER MODELS/ISOMERS ORGANI STRUTURES AND NAMING NOTE: This lab does not require safety glasses or lab coats. INTRODUTION Electron Dot Structures: Electron dot structures, also called Lewis dot diagrams, electron-dot diagrams or Lewis structures are diagrams that show the bonding between atoms of a molecule, and the lone pairs of electrons that may exist in the molecule. Electron dot structures are convenient way to represent the valence electrons in a molecule. For example, the Electron dot structure for l is formed by combining the Electron dot structure for a hydrogen atom with the Electron dot structure for a chlorine atom: Lewis Structure In Electron dot Structures, two dots between the symbols of two atoms represent a pair of electrons that is shared between the two atoms, that is, a bonding pair or covalent bond. Electrons that are not shared are called unshared pairs, lone pairs, or nonbonding electrons. ORGANI OMPOUNDS: a) arbon atoms are generally tetravalent. This means that carbon atoms in most organic compounds are bound by four covalent bonds to adjacent atoms. b) Organic molecules are three-dimensional and occupy space. The covalent bonds which carbon makes to adjacent atoms are at discrete angles to each other. These angles correspond to compounds which have triple bonds (1), double bonds (2), and single bonds (3), respectively.
Bond Angles: 180 o 120 o 109.5 o (1) (2) (3) The simplest hydrocarbons are alkanes in which all carbon-carbon bonds are single bonds and have the general formula: n 2n+2 (where n = 1, 2, 3 ). The simplest alkane is methane. Additional alkanes are listed below. n ompound Name 1 4 methane 2 3 3 ethane 3 3 2 3 propane 4 3 2 2 3 n-butane 5 3 ( 2 ) 3 3 n-pentane 6 3 ( 2 ) 4 3 n-hexane 7 3 ( 2 ) 5 3 n-heptane 8 3 ( 2 ) 6 3 n-octane 9 3 ( 2 ) 7 3 n-nonane 10 3 ( 2 ) 8 3 n-decane Example of a branched alkane. 3,3,4-trimethylhexane - 2 -
To name alkanes identify and circle the longest uninterrupted chain of carbon atoms and name it using the names listed in the table on page 1. In the example above the longest uninterrupted chain of carbon atoms contains 6 carbon atoms and thus is a derivative of hexane. Next number the carbons in the main chain so that the groups that are not part of the main chain receive the lowest possible number. Remember to give priority (the lowest number) to functional groups if present. Alkyl side groups, carbons not included in the longest chain, are named by comparing it to the corresponding alkane and replacing the ending ane with yl. For example if a hydrogen atom () is removed from methane ( 4 ) so that it can be attached to something else, it is referred to as a methyl group( 3 -). Typical alkyl groups are listed below. In the example the longest chain of carbon atoms contains three methyl groups and thus is trimethylhexane. Each group that is not part of the main chain must be clearly identified by its location on the main chain. These groups are named accordingly and given a number to indicate position. In the example above, the longest chain is numbered so that two of the methyl groups are on carbon 3 of the longest chain and the third is on carbon 4. Thus the compound is 3,3,4-trimethylhexane. (Note: Because - 3 -
there are three of the same groups as side chains in this example the prefix tri is used in front of the group name). ydrocarbons containing a carbon-carbon double bond(=) are known as alkenes ( n 2n ) and those with a carbon-carbon triple bond ( ) are alkynes ( n 2n-2 ). Alkenes and alkynes are named in the same manner as those for alkanes. owever, the ending ane of the corresponding alkane is replaced with ene for alkenes and yne for alkynes. The chain is numbered to give the lowest possible numbers to the carbons containing the double or triple bond. ydrocarbons which form a ring structure or cyclic structure of carbons are named by adding the prefix cyclo to the name and drawn using a line notation in which the hydrogen and carbon atoms are omitted and the end of each line and each corner represents a carbon atom and the corresponding number of hydrogens. See the examples below: cyclopropane cyclohexene Methylcyclopentane ISOMERS Organic compounds with the same molecular formula (same number of atoms per molecule) but different arrangement of atoms are known as isomers. onstitutional isomers (or structural isomers) are compounds with the same molecular formula but with different structural formula. Each compound has its own unique set of physical and chemical properties. While a molecular formula tells us the number and type of atoms present in a compound, it tells us nothing about the structure. The structural fomula shows the sequence in which the atoms are connected (also known as the bond order) and the bond types. For example, the molecular formula 4 10 can be represented by the two different structures for n-butane and methyl propane shown below: - 4 -
onsider also the molecular formula 4 6. Two structures which correspond to this formula are: 1-butyne 2-butyne Stereoisomers are isomers which have the atoms bonded in the same order but differ in their arrangement in space. o is/trans isomerism (geometric isomerism) occurs because of restricted bond rotation which freezes the molecule in their respective geometrical arrangements (i.e. no free rotation). is/trans isomerism occurs for certain alkenes, as seen below, and also for certain cycloalkanes. cis-2-butene trans-2-butene - 5 -
Alkanes have free rotation about the - single bonds. owever, in cycloalkanes, there is no rotation about the carbon atoms in the ring, which gives the carbon ring two distinct sides. onsider the following examples: hlorocyclopropane cis-1,2-dichlorocyclopropane trans-1,2-dichlorocyclopropane o hlorocyclopropane has just one isomer. o The compound 1,2-dichlorocyclopropane has two stereoisomers. The trans isomer has the chlorine atoms on opposite sides of the ring while the cis isomer has the chlorine atoms on the same side of the ring. In order to change one isomer into the other, one would be required to break bonds and reform new bonds. Aromatic compounds are hydrocarbons based on that of benzene ( 6 6 ) and typically have characteristic odors. Benzene is a six carbon ring with a single hydrogen atom covalently bonded to each carbon. See below. Benzene ring may be represented as follows. - 6 -
Typical aromatic compounds are listed below. Where X = 3 : toluene or methylbenzene 3 2 : ethylbenzene O: phenol N 2 : aniline O: benzaldehyde OO: benzoic acid When there are two substituents on the benzene ring, the ring is numbered giving the lowest number to the substituents. A common convention is to use the prefixes ortho, meta, and para. The prefix ortho (o) is for a 1,2-substituetion, meta(m) for a 1,3-, and para(p) for a 1,4-substitution. See example below for xylene or dimethylbenzene. o-xylene m-xylene p-xylene OR 1,2-dimehylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene Know how to answer questions like the following for the Molecular Models/Isomers quiz: 1. Be able to draw electron dot formulas for given compounds. 2. Know how to draw chemical structures using condensed and line notation. 3. Define the term structural isomer, and be able to give an example. 4. Name and draw chemical structures for alkanes, alkenes, alkynes and aromatic s using the IUPA nomenclature system. - 7 -