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Chapter 26 Structure of Organic.docx

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Pippa Lock

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Chapter 26: Structure of Organic Compounds Organic Compounds and Structures: An Overview  The simplest organic compounds are those of carbon and hydrogen – hydrocarbons – and the simplest hydrocarbon is methane  The three-dimensional structure of hydrocarbons can be shown by the structural formula, the condensed structural formula, the ball-and-stick model, the space filling model and the dashed-wedged line notation  Constitutional Isomerism in Organic Compounds o Compounds that have the same molecular formula, but different structural formulas are called isomers o Constitutional isomers have different bond connectivities and thus different skeletal structures o Ex. Butane vs. methylpropane  Nomenclature o Saturated hydrocarbons are those with all carbon-to-carbon bonds as single bonds (alkanes) o A side chain is an alkane with one hydrogen atom removed  The resulting group of atoms is called an alkyl group (methyl, propyl) o A primary carbon is attached to one other carbon atom  Carbon atoms at the end of an alkane chain are always primary carbons  The hydrogen atoms attached to a primary carbon atom are called primary hydrogen atoms  An alkyl group formed by removing a primary hydrogen is a primary group o A secondary carbon is attached to two other carbon atoms o A tertiary carbon is attached to three other carbon atoms o A quaternary is a carbon attached to four carbon atoms o The following rules enable us to name branched-chain hydrocarbons as long as we apply the rules in sequence  Select the longest continuous carbon chain in the molecule and use the hydrocarbon name of this chain as the base name  Consider every branch of the main branch to be a substituent alkyl group  Number the C atoms of the continuous base chain so that the substituents appear at the lowest numbers possible  Separate numbers from one another with commas, but no spaces and separate numbers from letters with hyphens  List the substituents alphabetically by name  When determining alphabetical order, the prefixes di-, tri-, sec- and tert- are ignored  Therefore tert-butyl precedes methyl in 4-tert-butyl-2- methylheptane  The prefix iso- is not ignored  Functional Groups o Organic compounds typically contain elements in addition to carbon and hydrogen o These elements occur as distinctive groupings of one or several atoms called functional groups o Compounds with the same functional group generally have similar chemical properties Alkanes  The essential characteristic of alkane hydrocarbon molecules is that they have only single covalent bonds  The bonds in these compounds are said to be saturated  Substances whose molecules differ only by a constant unit (-CH ) a2e said to form a homologous series  Members of such series usually have closely related chemical and physical properties  Intermolecular attractions between the straight-chain molecules are strongest and these molecules have the highest BP  Isomers with more compact structures have lower BP  Conformations o Conformations are different spatial arrangements that are possible in a molecule o One conformation can be converted into another by rotations about  bonds o Newman projections are used to represent the many different spatial arrangements of atoms that result from rotations around  bonds o A staggered conformation is when the carbon-hydrogen bonds in one –CH 3 group are positioned exactly halfway between those of the other –CH 3roup and the H atoms are located a maximum distance apart o The eclipsed conformation is when all the hydrogen atoms on the first carbon atom are directly in front of those on the second carbon atom o The eclipsed and staggered conformations represent two extremes and all the possible conformations in between are collectively called skew conformations o The difference in energy between the eclipsed and staggered conformations is called the rotational or torsional energy o The totally eclipsed conformation is highest in energy because in this conformation, the two largest constituents are eclipsed and interfere with each other  The interference or crowding of large substituents is called steric hindrance o The anti conformation is a staggered conformation when the substituents are farthest apart and it has the lowest energy of all the conformations o The other staggered conformations has the substituents to the left or and right of each other and is called the gauche conformation o All eclipsed conformations have higher energy than all staggered conformations  All the close atoms contribute to high energy o Conformation vs. Configuration  Conformation refers to the arrangement of atoms in a molecule that can be changed by simple rotation of single bonds, without breaking any bonds  Configuration is the permanent geometry of a molecule resulting from the spatial arrangement of its bonds  Must break bonds to change configuration  Preparation of Alkanes o The main source of alkanes is petroleum o In the presence of a metal catalyst such as Pt, Pb or Ni, unsaturated hydrocarbons, whether containing double or triple bonds, may be converted to alkanes by the addition of H atoms to the multiple bond systems o In another type of reaction, halogenated hydrocarbons react with alkali metals to produce alkanes of double the carbon content o Another method is to have alkali metal salts of carboxylic acids fuse with alkali metal hydroxides Isomers and Cycloalkanes  Skeletal or structural isomers have o The same molecular formula, but different connectivity o Different physical properties  Cycloalkanes o C Hn 2n o Cyclo-propane/butane/pentane/hexane.. o Molecules are not always flat  Ex. Cyclohexane can form the boat or chair conformation  The chair conformation is more favourable because the electron cloud is more spaced out o In cycloalkanes, there are equatorial substituents and axial substituents  Equatorial substituents are more favourable than axial substituents o Ring Strain 0  In propane, the bond angles of the carbon atoms are 109.5 C  In cyclopropane, the bond angles of the carbon atoms are 60 C  Also, the H atoms are eclipsed  Not very favourable, has high energy Alkenes and Alkynes  Hydrocarbons whose molecules contain some double or triple bonds between C atoms are said to be unsaturated  If the molecule has one double bond, they hydrocarbons are simple alkenes and have the general formula Cn 2n  Simple alkynes have one triple bond in their molecules and have the general formula Cn 2n-2  There are some modifications of rules required to name alkenes and alkynes o Select as the base chain the longest chain containing the multiple bond o Number the C atoms of the chain to place the multiple bond at the lowest possible number o Use the ending –ene for alkenes and –yne for alkynes  Ex. In the name 2-ethyl-1-pentene, the longest chain containing the multiple bond is a five-carbon chain o This is not a substituted hexane and is not a hexane  Stereoisomerism in Alkenes o The molecules 2-butene and 1-butene differ in the position of the double bond and are constitutional isomers o However, another type of isomerism is possible in 2-butene (cis and trans)  Cis is same side, trans is opposite side o They are stereoisomers because they have exactly the same constitution, but a different spatial arrangement of their atoms in space o A double bond between C atoms consists of the overlap of hybrid orbitals for form a  bond and the sideways overlap of p orbitals to form a  bond o Because of the  bond, rotation about a double bond is severely restricted o The two isomers cannot simple be converted to each other by twisting one end of the molecule through 180 so the two molecules are distinctly different o Because of differences in their molecular structures, the compounds have different physical properties  Preparation and Uses of Alkenes and Alkynes o General laboratory preparation of alkenes uses an elimination reaction, a reaction in which atoms are removed from adjacent positions on a carbon chain o In elimination reactions, a small molecule is produced and an additional bond is formed between the C bonds o Ex. H O2is eliminated in the following reaction o The principle alkene of the chemical industry is ethane (ethylene) o Its main use is in the manufacture of polymers o The simplest alkyne is ethyne (acetylene), which can be prepared from coal, water and limestone in a three-step process o Most other alkynes are prepared from acetylene by taking advantage of the C-H bond o In the presence of a very strong base such as sodium amide (NaNH ), the2amide anion removes the proton from acetylene to form ammonia and the salt sodium acetylide o Acetylide can then react with an alkyl halide such as CH B3 o The present use of acetylene is in the manufacture of other chemicals for polymer production, such as vinyl chloride, which is polymerized to polyvinyl chloride (PVC)  Naming the Stereoisomers of Highly Substituted Alkenes: The E, Z System of Nomenclature o The cis-trans nomenclature is not useful for naming highly substituted alkenes (alkenes with more than two different substituents) o An alternative system for naming such alkenes has been adopted by IUPAC, the E, Z system, also known as the Cahn-Ingold-Prelog rules o Used systematically to assign priorities to the substituents on the carbon atoms of the double bond o The stereochemistry about the double bond is assigned Z if the two groups of higher priority at each end of the double bond are on the same side of the molecule o If the two groups of higher priority are on opposite sides of the double bond, the configuration is denoted by an E o Look first at atoms directly bonded to double bond o Look at next set of atoms until a difference in priority is found Aromatic Hydrocarbons  Aromatic hydrocarbons have ring structures with unsaturation in carbon-carbon bonds  Most aromatic hydrocarbons are based on benzene, C H 6 6  Other examples of aromatic hydrocarbons are  Toluene and o-xylene are substituted benzenes and naphthalene and anthracene feature fused benzene rings  When rings are fused together, the resultant structure has two C and four H atoms fewer than starting structures  Characteristics of Aromatic Hydrocarbons o Highly flammable o All share two common features  They are planar, cyclic molecules  They have a conjugated bonding system, a bonding scheme among the ring atoms that consist of alternating single and double bonds (not always followed for hetero-aromatics or ions)  The system must extend throughout the ring and the  electron clouds associated with the double bonds must involve 4n+2 electrons, where n = 0, 1, 2… (Huckel Rule)  Don’t consider pi electrons if not in ring (ex. aldehyde) o If two benzene rings are connected by an alkyl group (12 pi e ), still aromatic because both rings are independently aromatic  Alkyl link doesn’t matter o Benzene and its homologues are similar to other hydrocarbons in being insoluble in water, but soluble in organic solvents o BP of aromatic hydrocarbons are slightly higher than those of alkanes of similar carbon content  Can be explained by planar structure and delocalized electron charge density of benzene, which decreases attractive forces between molecules  Symmetrical structure of benzene permits closer packing of molecules in crystalline state and results in higher MP o Two important aromatic groups are the phenyl and benzyl groups o A phenyl group is obtained when one of six equivalent H atoms of a benzene molecule is removed o A benzyl group is obtained by replacing one of the H atoms in a methyl group with a phenyl group  Heterocyclic compounds o Aromatic compounds can have heteroatoms in them o Huckel’s rule still applies o Ex. Pyridine  Lone pair of nitrogen is not counted toward the Huckel number because the sp hybridized orbital is perpendicular to the six 2p orbitals of the - system o Ex. Pyrrole  Because of double bonds, N gets rehybridized to sp 2  The two lone electrons are contributed to the aromatic system  Naming Aromatic Hydrocarbons o Use a numbering system for the C atoms in the ring o If the name of an aromatic compound is based on a common name other than benzene (ex. toluene), the characteristic substituent group (-CH 3or toluene) is assigned “1” on benzene ring o Otherwise, substituents are listed alphabetically and carbon atoms in the ring are numbered so that the substituents appear at the lowest numbers possible o The terms ortho, meta and para (o-, m-, p-) can be used when there are two substituents on the benzene ring o Ortho refers to substituents on adjacent carbons, meta to substituents with one carbon atom between them and para to substituents opposite one another on the ring Organic Compounds Containing Functional Groups  Alcohols and Phenols o Alcohols and phenols are characterized by the hydroxyl group –OH 3 o In alcohols, the hydroxyl group is bound to an sp hybridized carbon atom o If this C atom also has one R group (and two H atoms) bonded to it, the alcohol is a primary alcohol o If the C atom has two R groups (and one H atom), the alcohol is a secondary alcohol o If there are three R groups on the C atom (and no H atoms), the alcohol is a tertiary alcohol o The systematic naming of alcohol uses the suffix –ol o In phenols, the hydroxyl group is attached to the benzene ring o A molecule may have more than one –OH group o Those with two –OH groups are known as diols and those with more than two – OH groups are called polyls o Physical properties of aliphatic alcohols are strongly influenced by hydrogen bonding o As the chain length increases, however, the influence of the polar hydroxyl group on the properties of the molecule diminishes o The molecule becomes less like water and more like a hydrocarbon o As a consequence, low-molecular mass alcohols tend to be water soluble, whereas high-molecular mass alcohols are not o The BP and solubilities of phenols vary widely, depending on nature of other substituents on benzene ring o Preparation and Uses of Alcohols  Two methods by which alcohols can be synthesized are by the hydration of alkenes and the hydrolysis of alkyl halides  The first reaction is an example of an addition reaction and the second reaction is an example of a substitution reaction  In an addition reaction, one or m
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