Alkanes-2.pdf

Organic vs Inorganic Compounds Organic compounds (i.e. organic chemicals) are carbon-based compounds and are usually derived from living things (such as plants or animals). Inorganic compounds, on the other hand, are compounds that do not contain car...

Organic vs Inorganic Compounds Organic compounds (i.e. organic chemicals) are carbon-based compounds and are usually derived from living things (such as plants or animals). Inorganic compounds, on the other hand, are compounds that do not contain carbon atoms and can be found in minerals, rocks, and water. The easiest way to tell if a compound is organic or inorganic is by looking at its chemical formula. If it contains C or H atoms, it is likely to be an organic compound. ©2020 McGraw-Hill Education. 1 ©2020 McGraw-Hill Education. 2 ©2020 McGraw-Hill Education. 3  Chapter 2 Alkanes and Cycloalkanes: Introduction to Hydrocarbons (Left): Source: NASA/JPL-Caltech/USGS; (right): Source: NASA/JPL-Caltech/Space Science Institute ©2020 McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw-Hill Education. 4  Section 2.20 INTRODUCTION TO FUNCTIONAL GROUPS ©2020 McGraw-Hill Education. 5  Functional Groups Functional group - collection of atoms at a site within a molecule with a common bonding pattern - are structural units containing heteroatoms or multiple bonds 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 (See Figure 3.1) ©2020 McGraw-Hill Education. 6  Structures of some Common Functional Groups ©2020 McGraw-Hill Education. 7  Structures of some Common Functional Groups ©2020 McGraw-Hill Education. 8  Structures of some Common Functional Groups ©2020 McGraw-Hill Education. 9  Structures of some Common Functional Groups ©2020 McGraw-Hill Education. 10  Types of Functional Groups: 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 ©2020 McGraw-Hill Education. 11  Functional Groups with Carbon Singly Bonded to an Electronegative Atom Alkyl halide: C bonded to halogen (C-X) Alcohol: C bonded O of a hydroxyl group (C OH) Ether: Two C’s bonded to the same O (C O C) Amine: C bonded to N (C N) Thiol: C bonded to SH group (C SH) Sulfide: Two C’s bonded to same S (C S C) Bonds are polar, with partial positive charge on C (+) and partial negative charge (−) on electronegative atom ©2020 McGraw-Hill Education. 12  Functional Groups with Carbon Singly Bonded to an Electronegative Atom ©2020 McGraw-Hill Education. 13  Functional Groups with a Carbon– Oxygen Double Bond (Carbonyl Groups) Carbonyl groups are present in a large majority of organic compounds and biological molecules Behave similarly in many aspects Differ depending on the identity of the atoms bonded to the carbonyl-group carbon ©2020 McGraw-Hill Education. 14 Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups) Aldehyde: one hydrogen bonded to C=O Ketone: two C’s bonded to the C=O Carboxylic acid: ⎯OH bonded to the C=O Ester: C-O bonded to the C=O Amide: C-N bonded to the C=O Acid chloride: Cl bonded to the C=O Carbonyl C has partial positive charge (+) Carbonyl O has partial negative charge (-). 15  Functional Groups with a Carbon-Oxygen Double Bond (Carbonyl Groups) ©2020 McGraw-Hill Education. 16  Worked Example Identify the functional groups the molecules of methionine ©2020 McGraw-Hill Education. 17  Section 2.1 CLASSES OF HYDROCARBONS ©2020 McGraw-Hill Education. 18  Compounds of Carbon and Hydrogen Hydrocarbons are compounds containing only carbon and hydrogen Hydrocarbon compounds are classified according to their sources Aliphatic hydrocarbons are derived from fats and oils and contain mostly carbon-carbon single bonds Aromatic hydrocarbons are derived from essential oils and contain single and double carbon-carbon bonds ©2020 McGraw-Hill Education. 19  Structural Classification of Hydrocarbons Hydrocarbons can also be classified according to their structures Alkanes contain only C–C single bonds and C–H bonds Alkenes contain at least one C–C double bond Alkynes contain at least one C– triple bond Arenes contain a ring of alternating C–C and C=C bonds ©2020 McGraw-Hill Education. 20  Section 2.5 INTRODUCTION TO ALKANES: METHANE, ETHANE, AND PROPANE ©2020 McGraw-Hill Education. 21  Alkanes Alkanes have the general formula CnH2n+2 and consist of C–C and C–H bonds The simplest alkanes are methane, ethane, and propane. Boiling point increases with molecular weight All three molecules have tetrahedral geometry at their carbon atoms ©2020 McGraw-Hill Education. 22  Section 2.6 SP3 HYBRIDIZATION AND BONDING IN METHANE ©2020 McGraw-Hill Education. 23  Alkanes: A Geometric Quandary Valence bond theory suggests that bonds are derived from overlap of atomic orbitals. How can tetrahedral geometry be explained by overlap of s orbitals and p orbitals at 90 degree to one another? We make use of a mathematical device that explains the observed geometry Hybridization: atomic orbitals on the same atom are mixed to produce a new set of hybrid atomic orbitals ©2020 McGraw-Hill Education. 24  sp3 Hybridization To produce the four hybrid AOs on carbon needed for bonding in CH4, one 2s orbital and three 2p orbitals are mixed The resulting sp3 hybrids point to the vertices of a tetrahedron. ©2020 McGraw-Hill Education. 25  sp3 Hybridization-1 The four C–H bonds in CH4 are formed from overlap of the sp3 hybrid AOs with 1s AOs on the hydrogens ©2020 McGraw-Hill Education. 26  Structure and Hybridization of C2H4 Ethylene (H2CCH2) contains a double bond and trigonal planar geometry at each carbon atom The hybridization model can explain both the trigonal planar geometry and the C–C double bond The carbon atoms are sp2 hybridized ©2020 McGraw-Hill Education. 27  Structure and Hybridization in C2H2 Acetylene (HCCH) contains a triple bond and linear geometry at each carbon atom The hybridization model can explain both the linear geometry and the C–C triple bond The carbon atoms are sp hybridized ©2020 McGraw-Hill Education. 28  Alkane Isomers Straight-chain alkanes: Alkanes with C’s connected to no more than 2 other C’s Also called normal alkanes Branched-chain alkanes: Alkanes with one or more carbon atoms connected to 3 or 4 carbon atoms Isomers: Compounds that have the same number and kind of atoms but differ in the way the atoms are arranged ©2020 McGraw-Hill Education. 29  Counting and Naming Alkane Isomers There is not a general way to calculate the number of Table 2.1 The Number of Constitutionally Isomeric isomers associated with a Alkanes of Particular Molecular Formulas molecular formula Molecular formula Number of constitutional Isomers CH4 1 In simple cases, we can C2H6 1 enumerate possible C3H8 1 isomers by systematically C4H10 2 moving branches around C5H12 3 and changing their C6H14 5 composition C7H16 9 C8H18 18 The IUPAC nomenclature C9H20 35 system is a rigorous and C10H22 75 and systematic method for C15H32 4,347 naming alkanes C20H42 366,319 C40H82 62,491,178,805,831 ©2020 McGraw-Hill Education. 30  Section 2.15 IUPAC NOMENCLATURE OF UNBRANCHED ALKANES ©2020 McGraw-Hill Education. 31  Naming Linear Alkanes The IUPAC rules are used to name all organic compounds Alkanes are the foundation; we can think of organic compounds as derivatives of alkanes Linear alkane chains are named using a prefix for the number of carbons and the suffix –ane TABLE 2.2 IUPAC Names of Unbranched Alkanes Number of Number of Number of carbon atoms Name carbon atoms Name carbon atoms Name The first ten 1 Methane 11 Undecane 21 Henicosane prefixes are 2 Ethane 12 Dodecane 22 Docosane worth 3 Propane 13 Tridecane 23 Tricosane memorizing. 4 Butane 14 Tetradecane 24 Tetracosane 5 Pentane 15 Pentadecane 30 Triacontane 6 Hexane 16 Hexadecane 31 Hentriacontane 7 Heptane 17 Heptadecane 32 Dotriacontane 8 Octane 18 Octadecane 40 Tetracontane 9 Nonane 19 Nonadecane 50 Pentacontane 10 Decane 20 Icosane 100 Hectane ©2020 McGraw-Hill Education. 32  Section 2.17 ALKYL GROUPS ©2020 McGraw-Hill Education. 33  Alkyl Groups Partial structure that remains after the removal of one H from an alkane Not stable compounds, parts of larger compounds Named by replacing –ane ending of alkane with –yl ending –CH is methyl from methane (CH4) –CH2CH3 is ethyl from ethane (CH3CH3) ©2020 McGraw-Hill Education. 34  Beyond the Methyl Group What do we call substituents other than CH3? Other alkane substituents are known collectively as alkyl groups The names of alkyl groups follow the IUPAC rules for alkanes, but end in the suffix –yl Alkyl groups are classified by their substitution pattern at the point of attachment ©2020 McGraw-Hill Education. 35  Naming Alkyl Groups Importantly, the parent chain of an alkyl substituent must include the point of attachment NOT 2-propyl! Alkyl groups are often known by common names ©2020 McGraw-Hill Education. 36  Some Straight-Chain Alkyl Groups ©2020 McGraw-Hill Education. 37  Alkyl Groups Prefixes are used to represent the number of other carbon atoms attached to the branching carbon atom Symbol R is used in organic chemistry to represent a generalized organic group ©2020 McGraw-Hill Education. 38  Common Alkyl Groups ©2020 McGraw-Hill Education. 39  Section 2.15 IUPAC NOMENCLATURE OF BRANCHED ALKANES ©2020 McGraw-Hill Education. 40  Naming Branched Alkanes-4 Consider the structure at right. 1. Identify the longest continuous carbon chain 2. Identify the substituents, branches attached to the parent chain 3. Number the carbons of the longest chain such that the substituent at the first branching point gets the smallest number possible 4. Write the name of the compound 1. List substituents in alphabetical order, preceded by their number location 2. Write the parent chain as an alkane at the end of the name ©2020 McGraw-Hill Education. 41  Naming Alkanes International Union of Pure and Applied Chemistry (IUPAC) system of nomenclature Steps in naming in complex branched-chain alkanes Find parent hydrocarbon chain Identify the longest continuous chain of carbon atoms, use the name of that chain as the parent name ©2020 McGraw-Hill Education. 42  Section 2.18 IUPAC NAMES OF HIGHLY BRANCHED ALKANES ©2020 McGraw-Hill Education. 43  Highly Branched Alkanes The IUPAC rules are flexible enough to apply to any alkane. Consider 4-ethyloctane below When we add a methyl substituent to carbon 3… The name becomes 4-ethyl-3-methyloctane ©2020 McGraw-Hill Education. 44  Highly Branched Alkanes-1 Multiple copies of a substituent are denoted using the replicating prefixes di–, tri–, tetra–, etc. 4-ethyl-3,5-dimethyloctane These prefixes as well as italicized prefixes like sec- and tert- are ignored when alphabetizing Complications may arise when numbering… ©2020 McGraw-Hill Education. 45  The “First Point of Difference” Rule The two numbering schemes below both ensure that the substituent(s) at the first branching point have the number “2” The first scheme includes a second methyl group with the number “2”; the next substitutent in the second scheme is a 3-methyl. At the first point of difference, the first scheme has the smaller number (2-methyl versus 3-methyl). 2,2,6,6,7-pentamethyloctane 2,3,3,7,7-pentamethyloctane ©2020 McGraw-Hill Education. 46  Worked Example Give IUPAC names for the following compounds: a) b) ©2020 McGraw-Hill Education. 47  Section 2.19 CYCLOALKANE NOMENCLATURE ©2020 McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw-Hill Education. 48  Organic Compounds: Open-Chained or Cyclic Number of organic compounds contain rings of carbon atoms Example Prostaglandins Steroids ©2020 McGraw-Hill Education. 49  Naming Cycloalkanes Cycloalkanes or alicyclic compounds: Saturated cyclic hydrocarbons General formula (CnH2n) Can be represented using skeletal drawings ©2020 McGraw-Hill Education. 50  Cycloalkanes: Cyclic Alkanes Cycloalkanes are cyclic alkanes with the general formula CnH2n, where n > 2 Found in a wide variety of organic compounds Parent cycloalkanes are named using the prefix cyclo– Substituents are named in the usual way Where do we start numbering? ©2020 McGraw-Hill Education. 51  Naming Cycloalkanes Find the parent Count the number of carbons in the ring Count the number in the largest substituent ©2020 McGraw-Hill Education. 52  Numbering Cycloalkanes As for alkanes, we number cycloalkanes so that the substituent at the first point of difference gets the lowest number Typically, this means that the most substituted carbon is C1 ©2020 McGraw-Hill Education. 53  Naming Cycloalkanes Number the substituents Write the name ©2020 McGraw-Hill Education. 54  Cycloalkanes as Substituents Simple cycloalkanes bonded to longer alkyl chains are treated as cycloalkyl substituents Note that the cyclobutane ring is smaller than the attached pentane chain. ©2020 McGraw-Hill Education. 55  Worked Example Give IUPAC names for the following cycloalkanes a) ©2020 McGraw-Hill Education. 56  Cis-Trans Isomerism in Cycloalkanes Cycloalkanes are less flexible than open-chain alkanes Significantly lesser conformational freedom in cycloalkanes ©2020 McGraw-Hill Education. 57  Cis-Trans Isomerism in Cycloalkanes Cycloalkanes have two faces, when viewed edge- on, owing to their cyclic structure Top face and bottom face Isomerism is possible in substituted cycloalkanes Example - There are two different 1,2-dimethylcyclopropane isomers ©2020 McGraw-Hill Education. 58  Cis-Trans Isomerism in Cycloalkanes Stereoisomerism: Compounds which have their atoms connected in the same order but differ in 3- D orientation Stereochemistry: Term used to refer to the three- dimensional aspects of chemical structure and reactivity ©2020 McGraw-Hill Education. 59  Cis-Trans Isomerism in Cycloalkanes Cis-trans isomers: Stereoisomers that differ in their stereochemistry about a ring or double bond Common occurrence in substituted cycloalkanes and several cyclic biological molecules ©2020 McGraw-Hill Education. 60  Worked Example Draw the structures of the following molecules: a) trans-1-Bromo-3-methylcyclohexane ©2020 McGraw-Hill Education. 61  Worked Example Draw the structures of the following molecules: a) trans-1-Bromo-3-methylcyclohexane Solution: a) trans-1-Bromo-3-methylcyclohexane ©2020 McGraw-Hill Education. 62  Stability of Cycloalkanes: Ring Strain Angle strain: Induced in a molecule when bond angles are forced to deviate from the ideal 109°tetrahedral value Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering Torsional strain - Caused due to eclipsing of bonds between neighboring atoms Steric strain - Caused due to repulsive interactions when atoms approach each other too closely ©2020 McGraw-Hill Education. 63  Newman Projection A Newman projection is a drawing that helps visualize the 3-dimensional structure of a molecule. This projection most commonly sights down a carbon-carbon bond, making it a very useful way to visualize the stereochemistry of alkanes. ©2020 McGraw-Hill Education. 64  Newman Projection ©2020 McGraw-Hill Education. 65  Newman Projection ©2020 McGraw-Hill Education. 66  Stability of Cycloalkanes: Ring Strain Larger rings have many more possible conformations than smaller rings More difficult to analyze ©2020 McGraw-Hill Education. 67  Newman Projection ©2020 McGraw-Hill Education. 68  Newman Projection ©2020 McGraw-Hill Education. 69  Section 2.21 SOURCES OF ALKANES AND CYCLOALKANES ©2020 McGraw-Hill Education. 70  Alkanes from Petroleum Petroleum deposits are a very common source of alkanes and other hydrocarbons Crude oil can be distilled over a range of temperatures to separate it into pure compounds ©2020 McGraw-Hill Education. 71  Biological Sources Organic compounds, including alkanes, are commonly found in biological organisms The linear pentacosane, CH3(CH2)23CH3, is present in the waxy covering of many insects Hentriacontane, CH3(CH2)29CH3, is found in beeswax and other natural waxes Biological compounds based on cycloalkanes are rare, but do appear in some contexts ©2020 McGraw-Hill Education. 72  Section 2.22 PHYSICAL PROPERTIES OF ALKANES AND CYCLOALKANES ©2020 McGraw-Hill Education. 73  Intermolecular Forces in Alkanes Alkanes consist of nonpolar C–C bonds and very weakly polarized C–H bonds. Hence, they are nonpolar overall However, alkane molecules usually exhibit instantaneous dipoles due to asymmetric distributions of electrons at any point in time Instantaneous dipoles can induce a dipole in a nearby molecule, resulting in attractive forces The instantaneous dipole–induced dipole force is called a van der Waals interaction or London dispersion force ©2020 McGraw-Hill Education. 74  van der Waals Forces There are three types of van der Waals forces: 1. dipole–dipole (including hydrogen bonding) 2. dipole/induced-dipole 3. induced-dipole/induced-dipole These forces are electrical in nature, and in order to vaporize a substance, enough energy must be added to overcome them. Most alkanes have no measurable dipole moment, and therefore the only van der Waals force to be considered is the induced-dipole/induced-dipole attractive force. ©2020 McGraw-Hill Education. 75  Boiling Points Boiling points of alkanes increase with molecular weight Branched alkanes have lower boiling points than linear alkanes Branched alkanes have smaller surface area than isomeric linear alkanes. van der Waals forces are weaker in branched alkanes as a result. ©2020 McGraw-Hill Education. 76  Melting Points Solid alkanes are typically soft and waxy substances with relatively low melting points Molecules in the solid pack together closely Melting points points of alkanes also increase with molecular weight Branched alkanes have lower melting points than linear alkanes ©2020 McGraw-Hill Education. 77  A Plot of Melting and Boiling Points Versus Number of Carbon Atoms for the C1–C14 ©2020 McGraw-Hill Education. 78  Solubility in Water Alkanes are nonpolar substances characterized by very weak van der Waals interactions van der Waals interactions of alkanes with water cannot overcome the strength of hydrogen bonds in water Hence, alkanes are not generally soluble in water The Hydrophobic Effect: alkane molecules cluster together and exclude water molecules ©2020 McGraw-Hill Education. 79  Section 2.23 CHEMICAL PROPERTIES: COMBUSTION OF ALKANES ©2020 McGraw-Hill Education. 80  Alkanes React with Oxygen Alkanes are highly reduced forms of organic compounds, which contain only C–C and C–H bonds They react vigorously and exothermically with O2 gas in combustion reactions to form CO2 and H2O    H = H products −H reactants ©2020 McGraw-Hill Education. 81  Heats of Combustion TABLE 2.3 Heats of Combustion (−∆H°) of Representative Alkanes −∆H° −∆H° Compound Formula kJ/mol kcal/mol Unbranched alkanes Hexane CH3(CH2)4CH3 4,163 995.0 Heptane CH3(CH2)5CH3 4,817 1,151.3 Octane CH3(CH2)6CH3 5,471 1,307.5 Nonane CH3(CH2)7CH3 6,125 1,463.9 Decane CH3(CH2)8CH3 6,778 1,620.1 Undecane CH3(CH2)9CH3 7,431 1,776.1 Dodecane CH3(CH2)10CH3 8,086 1,932.7 Hexadecane CH3(CH2)14CH3 10,701 2,557.6 2-Methyl-branched alkanes 2-methylpentane (CH3)2CHCH2CH2CH3 4,157 993.6 2-methylpentane (CH3)2CH(CH2)3CH3 4,812 1,150.0 2-methylpentane (CH3)2CH(CH2)4CH3 5,466 1,306.3 ©2020 McGraw-Hill Education. 82  Branching and Stability Intramolecular forces affect heats of combustion of isomeric alkanes Branched isomers generally have less exothermic heats of combustion, indicating greater stability Attractive intramolecular forces in the branched isomer stabilize it relative to n-octane. ©2020 McGraw-Hill Education. 83