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Alkanes Hydrocarbons Are Organic Structures That Contain Only Carbon

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Alkanes Hydrocarbons Are Organic Structures That Contain Only Carbon Alkanes Hydrocarbons are organic structures that contain only carbon and hydrogen Alkanes are the simplest hydrocarbons with a molecular formula of CnH2n+2 CH4 methane The compound has a root name indicating the number of carbons and the –ane suffix What is the structure of methane? Structure of organic compounds in general indicate how properties are derived If we know the functional groups present and the structure of the compound, we can often predict the properties of a compound without needing to memorize every possible compound Structure of Methane As learned, molecules are made by combining atomic orbitals to form molecular orbitals Using the outer shell orbitals of methane, the compound results from combining the 2s, 2px, 2py and 2pz orbitals of carbon with the four 1s orbitals of hydrogen The electronic configuration for atomic carbon thus has a lower energy 2s orbital and three degenerate 2p orbitals that are each orthogonal to the others The valence electrons will thus have 2 in the lower energy 2s orbital and 1 in each of the degenerate 2px and 2py orbitals To form the methane molecule, therefore the 1s orbital of each hydrogen must form bonds with the orbitals that have electrons This bonding model has many problems: 1) implies different energy and bond length of C-H bonds 2) Two of the C-H bonds must have a 90˚ bond angle 3) Have too many electrons in an orbital Hybridization Model for Bonding Instead of using atomic orbitals for bonding, a different model considers first hybridizing the atomic orbitals to form “hybridized” orbitals Same rules apply for combining atomic orbitals to form hybrid orbitals 1) Get same number of hybridized orbitals as starting atomic orbitals used to form hybrid 2) Shape of hybridized orbitals is obtained by the mathematical addition of the wave functions for the atomic orbitals The name (designation) of hybridized orbitals merely refers to the number and type of atomic orbitals used in the formation sp Orbital Combine one s orbital with one p orbital Notice the relative shape difference between bonding and antibonding lobes -Allows more overlap! If the orbitals are subtracted then an identical hybridized orbital is obtained directed 180˚ from the first Bonds formed from the two sp hybridized orbitals will thus have a 180˚ bond angle sp2 Hybridization -Can also hybridize by combining one s orbital with two p orbitals (would allow formation of three covalent bonds – one from each sp2 hybridized) Look in the x-y plane, three sp2 hybridized orbitals pz is coming in and out of the plane All three sp2 orbitals are in the same plane (large lobe used for bonding 120˚ apart from one another) sp3 Hybridization To form four equivalent bonds carbon can hybridize all of its valence orbitals (three p and one s to form four sp3 hybrids) The four sp3 hybridized orbitals have a bond angle of 109.5˚ Forms a tetrahedral geometry All bonds can thus have the same bond length and angle, unlike the model using atomic orbitals Hybridization Model for Bonding When a hybridized orbital is used to form a bond with an atom, a new bonding and antibonding molecular orbital are formed These bonds have the electron density cylindrically symmetric about the internuclear axis Bonds that are symmetric about the internuclear axis are called sigma (σ) bonds Sigma bonds and lone pair of electrons (if they are not involved in resonance) use hybridized orbitals for the electrons When 2nd row atoms have the same substituents, they use sp hybridization for two bonding orbitals, sp2 hybridization for 3 bonding orbitals, and sp3 hybridization for 4 bonding orbitals Knowing the structure thus allows chemists to predict the hybridization and also the geometry for the compound Bonding in Unsymmetrical Compounds In methane there are 4 identical bonds between carbon and each of the four hydrogens The carbon atom thus adopts a sp3 hybridization and each H-C-H bond angle is 109.5˚ for a perfect tetrahedron geometry When one of the C-H bonds is replaced with a different atom, however, the perfect tetrahedron geometry is no longer present (The C-Br bond length is obviously longer than the C-H bonds, thus not a tetrahedron) We still approximate the carbon as being sp3 hybridized, it is very close as seen by geometry, but we realize this is an approximation Variable Hybridization As seen, the hybridization affects the geometry of a compound Atomic orbitals need not be “hybridized” in integer numbers, need not add exactly one s orbital with 2 p orbitals to yield exactly a sp2 hybridized orbital As the amount of s and p orbital ratios are changed, the geometry changes Pure s sp sp2 sp3 Pure p %s 100 50 33 25 0 %p 0 50 67 75 100 Bond < ~ 180˚ 120˚ 109.5˚ 90˚ As %p increases in a hybridized bond, the bond angle decreases As %s increases in a hybridized bond, the electrons are held closer to the nucleus (since s orbitals are closer to the nucleus on time average than p orbitals) The geometry is thus intimately related to the hybridization of the atom Drawing Organic Compounds Organic chemists use a wedge and dash line system to designate stereochemistry Wedge line – object is pointing out of the plane Dash line – object is pointing into the plane H H H H To draw a tetrahedral carbon: 1) Make a V with an angle approximately at 109.5˚ 2) Place the wedge and dashed lines in the obtuse angle space Common errors: 1) placing dashed and wedge lines in acute space 2) Placing either two bonds as wedge or dashed with two bonds in plane 3) Placing dashed and wedge bonds on opposite sides of bonds in plane Reactive Intermediates Methyl groups (CH3) if not attached to a fourth atom can form reactive intermediates (not stable structures, but rather intermediates along a reaction trajectory) Methyl anions Methyl anions are formed if a hydrogen is abstracted from methane by a base 3 H B carbon anions are approximately sp hybridized, Lone pairs go into hybridized orbitals H H H H H H (if not involved in resonance) Methyl cations Methyl cations have only 6 electrons in the outer shell of carbon H Carbocations are sp2 hybridized, H H H H the H-C-H bond angle is 120˚ H H Methyl radicals Methyl radicals have 7 electrons in the outer shell of carbon H 2 H Radicals are also assumed sp hybridized, H H H H H the H-C-H bond angle is 120˚ Conformational Analysis of Alkanes -Physical properties of molecules are determined by intermolecular forces (forces between molecules) -The internal structure of a given molecule can affect the energy due to sterics (intramolecular interactions) Conformer: different arrangements in space resulting from the rotation of bonds (bonds are not broken when interconverting between conformers) Consider Methane H H H H No conformers possible; methane has a given energy value that does not change (any rotation about the equivalent C-H σ bonds yields the same structure in three-dimensions) *this is not the case with any higher hydrocarbon homologue Conformational Analysis of Ethane H H H H H H H Rotate 60˚ Rotate 60˚ H H H H H H H H H H H Structures have different energy due to different arrangements in space (hydrogens have different spatial arrangements in different conformers) Newman Projections - Convenient way to view conformational analysis Which hydrogens are attached to front View down H H H or back carbon? H H C-C bond H H H H C H H H H H H H H H In order to distinguish the front atom from the back atom in a Newman projection, the substituents are attached to a point for the front carbon and to a circle for the back carbon In Newman Projections change view by looking down one carbon-carbon bond To Draw Newman Projections 1) Determine which bond is being considered 2) Determine which atom is front atom of bond being considered 3) The substituents attached to the front atom are drawn to a point, the substituents attached to the back atom are drawn to a circle 4) The relative angles and orientation of the substituents are maintained Consider butane 1) Look down the C2-C3 bond of butane CH3 CH3 2) Assign front and back atom of bond H H H3C H H 3) 2 hydrogens and one CH group are attached Front atom of 3 CH3 C2-C3 bond to both front and back carbons 4) Draw Newman projection maintaining bond orientation Newman projections of ethane conformations Substituents are Substituents are as “staggered” as “eclipsing” each other possible H H H H H H Rotate 60˚ Rotate 60˚ H H H H H H H H H H H H Substituents have returned to staggered eclipsed a “staggered” conformation Newman projections demonstrate energetic and spatial interactions of conformers Eclipsed conformations are higher in energy One cause is the sterics As the substituents that are eclipsed become larger, the energy of the conformer raises H3C H3C CH3 CH3 staggered eclipsed Consider the space filling volume of atoms Conformational Energy Diagram for Propane The energy difference between staggered and eclipsed conformations is larger for propane versus ethane due to the greater steric interaction between larger methyl and hydrogen Different Types of Interactions Arise with Larger Carbon Frameworks Consider butane looking down the C2-C3 bond totally eclipsed eclipsed CH H3C 3 H CH3 H CH3 H H H H “totally eclipsed” H CH3 H H H H conformation (which has H3C H largest groups eclipsing each other) is higher in energy than other eclipsed conformations y g r e n E CH3 “gauche” conformation is CH3 H CH3 H H higher in energy than anti H H (both are “staggered” H H H CH3 conformations) anti gauche -60˚ 0˚ 60˚ 120˚ 180˚ 240˚ torsional angle Naming Conventions for Alkanes Straight Chain Alkanes The
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