Organic Chemistry I: Saturated Hydrocarbons and their Stereochemistry PDF

Course name: Organic I Chapter II: Saturated Hydrocarbons and their stereochemistry 1  Outcomes Covered CLO2: recognize the conformational and configurational isomers. CLO3: Summarize physical and chemical properties of hydrocarbons and orga...

Course name: Organic I Chapter II: Saturated Hydrocarbons and their stereochemistry 1  Outcomes Covered CLO2: recognize the conformational and configurational isomers. CLO3: Summarize physical and chemical properties of hydrocarbons and organohalides. CLO4: predict the reaction mechanism of addition, elimination and substitution reactions. Duration to teach this chapter: FIVE Hours (TWO n Half weeks) Topics to be covered 2.1 Functional groups. 2.2 Alkane and alkane isomers. 2.3 Alkyl groups. 2.4 Naming alkanes and cycloalkanes. 2.5 Properties of Alkanes. 2.6 Conformations of ethane. 2.7 Conformations of other alkanes. 2.8 Stability of Cycloalkanes: Ring strain. 2.9 Conformations of Cycloalkanes and cyclohexane. 2 2.1 Functional Groups Functional group - Atom or group of atoms at a site that have a characteristic behavior in all molecules wherever it occurs The group reacts in a typical way, generally independent of the rest of the molecule For example, the double bonds in simple and complex alkenes react with bromine in the same way Loading… 3 Functional Groups with Multiple Carbon–Carbon Bonds Alkenes have a C=C double bond Alkynes have a C C triple bond Arenes have special bonds that are represented as alternating single and double C-C bonds in a six- membered ring. 4 Functional Groups with Carbon Singly Bonded to an Electronegative Atom Loading… 5 Groups with a Carbon–Oxygen Double Bond (Carbonyl Group) 6 Survey of Functional Groups 7 Survey of Functional Groups 8 2.2 Alkanes and Alkane Isomers Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups) Connecting carbons can lead to large or small molecules The formula for an alkane with no rings in it must be CnH2n+2 where the number of C’s is n Alkanes are saturated with hydrogen (no more can be added They are also called aliphatic compounds 9 Alkane Isomers CH4 = methane, C2H6 = ethane, C3H8= propane The molecular formula of an alkane with more than three carbons can give more than one structure (called as isomers) C4 (butane) = butane and isobutane C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes 10 Constitutional or Structural isomers Isomers that differ in how their atoms are arranged in chains are called constitutional isomers Compounds other than alkanes can also be constitutional isomers of one another (position and functional) They must have the same molecular formula to be isomers Loading… 11 Condensed Structures of Alkanes We can represent an alkane in a brief form or in many types of extended form A condensed structure does not show bonds but lists atoms, such as CH3CH2CH2CH3 (butane) CH3(CH2)2CH3 (butane) 12 13 2.3 Alkyl Groups Alkyl group – remove one H from an alkane (a part of a structure) General abbreviation “R” (for Radical, an incomplete species or the “rest” of the molecule) Name: replace -ane ending of alkane with -yl ending -CH3 is “methyl” (from methane) -CH2CH3 is “ethyl” (from ethane) 14  -H Alkane Alkyl Group CH4 CH3- methane methyl H3C-CH3 H3C-CH2- etahne ethyl H3C-CH-CH3 H3C-CH2-CH3 H3C-CH2-CH2- iso propyl propane propyl or n-propyl H3C-CH2-CH-CH3 H3C-CH2CH2-CH3 H3C-CH2CH2-CH2- sec-butyl butane butyl or n-butyl CH3 CH3 H2C H3C C H3C CH H3C CH iso butyl tert. butyl CH3 isobutane CH3 CH3 15 Types of Alkyl groups Classified by the connection site (See Figure 3.3) a carbon at the end of a chain (primary alkyl group) a carbon in the middle of a chain (secondary alkyl group) a carbon with three carbons attached to it (tertiary alkyl group) 16 2.4 Naming Alkanes Follows specific rules Find parent hydrocarbon chain Carbons in that main chain are numbered in sequence Substituents are identified numbered Write compound name is single word Name a complex substituents as though it were a compound itself See specific examples in text Try ThomsonNow Organic Interactive from p. 90 of your text 17  1. Select the longest continuous chain of carbon atoms as the parent chain and name the hydrocarbon. 2. Number the carbon atoms in the longest continuous chain in such a way as to give lowest possible number to carbons atoms carrying substituents. 3. Name the substituent. Indicate its position by the number of the carbon atom to which it is attached. 4. Prefix the position number and name of the substituent onto the parent name. The whole name is written as one word. Note that the number and name of the substituent are separated by a hyphen. 5. If identical substituents are present more than once in the molecule, then use prefixes di-, tri-, tetra-, penta- etc. Position of each substituent is indicated by a separate number. 6. When two or more different substituents are present, their names are arranged in alphabetic order and added to the name of the parent alkane, again as one word.  2- Methyl Butane Position of Attached alkyl group Longest chain alkyl group 1 2 3 4 CH3 CH CH2 CH3 2-methylbutane CH3 CH3 CH3 CH CH 5 3 H3C 4 C 2 CH3 H2 1 2,4-dimethylpentane  Naming Cycloalkanes Cycloalkanes are saturated cyclic hydrocarbons. Have the general formula (CnH2n) Naming Cycloalkanes Find the parent. # of carbons in the ring. Number the substituents 22 2.5 Properties of Alkanes Alkanes are called paraffins (low affinity compounds) because they are saturated and do not prefer to undergo reactions. Physical Properties Boiling points and melting points increase as size of alkane increases Dispersion forces increase as molecule size increases, resulting in higher melting and boiling points 23 Chemical Properties Alkanes are called paraffins (low affinity compounds) because they are saturated and do not prefer to undergo reactions. They undergo two types of reactions Combustion (complete burning in excess oxygen / air) Alkanes burn in oxygen, producing carbon dioxide, water, and heat. Alkane + O2 CO2 + H2O + heat 24  Free Radical Substitution Reactions They react with Cl2 in the presence of light to replace H’s with Cl’s (not controlled). This reaction follows free radical mechanism. A radical is highly reactive because it contains an atom with an odd number of electrons in its valence shell. 2/6/2023 Prepared By: KAB 25 Generation of Radical and the ways it can react: 26 Chemical equation showing overall reaction of methane with one mole of chlorine Free Radical Reaction Mechanism It takes place in three steps. 1) Initiation Irradiation with UV light begins the reaction by breaking the relatively weak Cl-Cl bond to give reactive chlorine radical 2) Propagation Once produced, a reactive chlorine radical collides with a methane molecule in a propagation step, abstracting a hydrogen atom to give HCl and a methyl radical (·CH3). 3) Termination two radicals might collide and combine to form a stable product 2/6/2023 Prepared By: KAB 27 Physical Properties Boiling points and melting points increase as size of alkane increases Dispersion forces increase as molecule size increases, resulting in higher melting and boiling points 28 2.6 Conformations of Ethane Stereochemistry concerned with the 3-D aspects of molecules bonds are cylindrically symmetrical Rotation is possible around C-C bonds in open-chain molecules Loading… 29 Conformers Conformation- Different arrangement of atoms resulting from bond rotation Conformations can be represented in 2 ways: 30 Torsional Strain We do not observe perfectly free rotation There is a barrier to rotation, and some conformers are more stable than others Staggered- most stable: all 6 C-H bonds are as far away as possible Eclipsed- least stable: all 6 C-H bonds are as close as possible to each other 31 32 2.7 Conformations of Other Alkanes The eclipsed conformer of propane has 3 interactions: two ethane-type H-H interactions, and one H-CH3 interaction 33  Conformations of Other Alkanes Conformational situation is more complex for larger alkanes Not all staggered conformations has same energy, and not all eclipsed conformations have same energy. 34 Conformations of Butane Anti conformation- methyl groups are 180˚ apart Gauche conformation- methyl groups are 60˚ apart Which is the most energetically stable? 35 Steric Strain Steric strain- repulsive interaction occurring between atoms that are forced closer together than their atomic radii allow 36 37 2.8 Stability of Cycloalkanes: Ring Strain Rings larger than 3 atoms are not flat Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering Larger rings have many more possible conformations than smaller rings and are more difficult to analyze Stability of Cycloalkanes: The Baeyer Strain Theory Baeyer (1885): since carbon prefers to have bond angles of approximately 109°, ring sizes other than five and six may be too strained to exist Rings from 3 to 30 C’s do exist but are strained due to bond bending distortions and steric interactions 39  Summary: Types of Strain Angle strain - expansion or compression of bond angles away from most stable Torsional strain - eclipsing of bonds on neighboring atoms Steric strain - repulsive interactions between nonbonded atoms in close proximity 40 2.9 Conformations of Cycloalkanes Cyclopropane 3-membered ring must have planar structure Symmetrical with C–C–C bond angles of 60° Requires that sp3 based bonds are bent (and weakened) All C-H bonds are eclipsed 41  Bent Bonds of Cyclopropane In cyclopropane, the C-C bond is displaced outward from internuclear axis 42 Cyclobutane Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens Cyclobutane is slightly bent out of plane - one carbon atom is about 25° above The bend increases angle strain but decreases torsional strain 43 Cyclopentane Planar cyclopentane would have no angle strain but very high torsional strain Actual conformations of cyclopentane are nonplanar, reducing torsional strain Four carbon atoms are in a plane The fifth carbon atom is above or below the plane – looks like an envelope 44 Conformations of Cyclohexane Substituted cyclohexanes occur widely in nature The cyclohexane ring is free of angle strain and torsional strain The conformation is has alternating atoms in a common plane and tetrahedral angles between all carbons This is called a chair conformation 45 How to Draw Cyclohexane 46 Axial and Equatorial Bonds in Cyclohexane The chair conformation has two kinds of positions for substituents on the ring: axial positions and equatorial positions Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorial hydrogens near the plane of the ring 47 Axial and Equatorial Positions Each carbon atom in cyclohexane has one axial and one equatorial hydrogen Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement 48 Drawing the Axial and Equatorial Hydrogens 49 Conformational Mobility of Cyclohexane Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip https://www.youtube.com/watch?v=bPLREpfZ63I 50