Chapter 2 Alkanes and Cycloalkanes Section 1 Alkanes Section 2 Cycloalkanes

Definition and Classification of Hydrocarbons Alicyclic CH3CH3 Alkanes Saturated Unsaturated Aliphatic ~ Alkenes Alkynes CH2=CH2 CH ≡CH Hydrocarbon is only composed of carbon and hydrogen. Aromatic ~

Section 1 Alkanes Structure Nomenclature Properties Alkane is one kind of hydrocarbon in which all of the carbon are bound to each other and hydrogen by single (s) bonds. Structure Nomenclature Properties

Ⅰ. Structure 1. sp3 hybridization C: 1s2; 2s22p2 sp3 tetrahedral shape

Staut model of methane CH4 takes a tetrahedron shape. Ball-stick model

sp3 hybridization and structure of methane s-sp3 s bond by head to head overlap (see remark) 109.5o sp3 hybridization and structure of methane σbond:It is formed by the overlap of two atomic orbitals along the line joining the two nuclei of the atoms involved (bond-axis). It is symmetrically about the axis joining the nuclei of the atoms (bond-axis). Nature ofσbond: ① symmetrical about the bond-axis; ② firm and not easy to break; ③σ bond can rotate around the bond-axis

C－Cσbond formed by head to head bond : rotate freely ； bigger overlap degree; higher stability, lower polarity. Ethane (CH3CH3) C－Cσbond formed by head to head near 109.5oC(tetrahedral) 110pm 154pm s bond

Bonding of alkanes All carbons are hybridized by sp3 All bonds are σ-bonds All bond angles are about 109°28’ Butane(C4H10) Pentane(C5H12)

General formula: CnH2n+2 CH4, CH3CH3, CH3CH2CH3, CH3CH2CH2CH3…… Homolog has similar chemical properties with different reacting rate, but their physical properties vary with the number of carbon atoms. The first member of homologous series often has special property.

3. Isomerism of alkanes Isomerism Carbon-chain ~ Constitutional Functional ~ The atoms are linked together in different way Isomerism Positional ~ Cis-trans ~ Configurational ~ Stereo- isomerism Enantio- merism Conformational ~

(1) Carbon-chain isomerism: n-Pentane iso-Pentane neo-Pentane

Q 1 Write structural formulas or skeletal structures (simplified line structures) for the isomeric hexanes (C6H14).

A: (b) same, (c) isomer, and (d) same. Q 2 Identify as same or isomer the relationship between structure (a) and structure (b), (c), and (d). A: (b) same, (c) isomer, and (d) same.

Classification of C and H C : be grouped by the number of carbon atoms it attached. primary carbon，1° secondary carbon，2° tertiary carbon ，3° quaternary carbon ，4° 1° 1° 2° 3° 2° 4° 1° 1° 1°

Notice: H: Primary H ( 1º H ): those attach to 1º carbon Secondary H ( 2º H ): those attach to 2º carbon Tertiary H ( 3º H ): those attach to 3º carbon H: Notice: No “ 4°” H

Which of the following compound contains 1°, 2°, 3°and 4°carbon atoms? A、2，2，3-trimethylbutane B、2，2，3-trimethylpentane C、2，3，4-trimethylpentane D、3，3-dimethylpentane Answer: B

(2) Conformational isomerism The bonding arrangements of atoms remains constant, but the relationship of the atoms in space differs as a result of rotation around a single bond are called conformations isomerism. Conformational isomers of ethane

(A) The characteristics of conformations 1. interconverting rapidly and easily at room temperature. 2. an infinite number of conformations of ethane. 3. very little difference in physical properties, but probably great difference of some molecules in biochemistry .

(B) Representation of conformational isomers represented in three ways: perspective formula, sawhorse formula and Newman projections. perspective formula

Sawhorse representations view the C—C bond from a C—C bond oblique angle. sawhorse formula

Newman projection structures view the C-C bond end-on along the axis of connection. Staggered Eclipsed Newman projection

(C) Energy analysis of conformations 249 306 227 289 < the sum of V’s radius repulsion will increase greatly ＞≈ the sum of V’s radius attract each other weakly higher energy lower energy

Energy curve of ethane conformations The low energy conformations is termed as conformer.

(D) Conformations of butane The conformation is more complex.

Q: Which one is the most stable, which one is the least?

Energy curve of butane conformation (~70%) (~30%) Energy curve of butane conformation

At room temperature, butane has many conformational isomers Notice: At room temperature, butane has many conformational isomers The most stable one is staggered The most unstable one is eclipsed Conclusion The energy of four typical conformations: Anti-conformation Gouche conformation Eclipsed(partial) conformation Eclipsed conformation < < <

in zigzag chain

Q: How many conformations of butane are there? A. 2 B. 4 C. >4 D. infinite

Nomenclature

1. Common nomenclature CH3CH2CH2CH3 CH3CHCH3 CH3 n- iso Butane butane carbon atoms < 6. Straight chain : normal alkanes. contains iso group (CH3)2CH-, : iso-alkane. CH3CH2CH2CH3 CH3CHCH3 CH3 n- Butane butane iso n-Pentane Isopentane Neopentane

A chemical name has 3 parts in the IUPAC system. 2. IUPAC nomenclature A chemical name has 3 parts in the IUPAC system. Prefix Parent Suffix Where; what; how many (are the substituents?) What family? How many Carbons?

(1) Unbranched alkanes (1~10): Formula Name Formula Name CH4 methane C6H14 hexane C2H6 ethane C7H16 heptane C3H8 propane C8H18 octane C4H10 butane C9H20 nonane C5H12 pentane C10H22 decane C11H22 undecane C12H26 dodecane C13H28 tridecane C14H30 tetradecane C15H32 pentadecane C20H42 eicosane

Alkyl group: one H is removed from an alkane. (2) Alkyl groups: ( R— ) R—H An alkane Alkyl group: one H is removed from an alkane. Methyl (Me) Ethyl (Et) Propyl (Pr) Isopropyl (i-Pr)

Butyl (Bu) (i-Bu) sec-Butyl (s-butyl,s-Bu) Isobutyl tert-Butyl (t-butyl,t-Bu) Neopentyl tert-Pentyl, t-Pentyl (t-amyl)

(3) Branched alkanes: Parent: octane Step 1: Find the parent. Find the longest continuous chain of carbon atoms; this chain determines the base name for the alkane. Parent: octane

When two chains of equal length compete for selection as the base chain, choose the one with more substituents. x 7 6 5 Parent: heptane

Step 2: Number each carbon atom in the parent chain from the end of the chain nearest the branching . Parent: heptane

Step 3: Name the alkyl groups, locate their positions by the numbers and write the name as a single word, using hyphens to separate the various prefixes and commas to separate numbers. If two or more different side chains are present, list them in order of alphabet. On C-5, CH3- On C-3, CH3CH2- 3-Ethyl - 5-methyl octane

1 2 3 4 5 2,3,4-Trimethylpentane If two or more identical side chains are present, use di-, tri-, tetra-, and so forth Prefixes: di=2 tri=3 tetra=4 penta=5 hexa=6 hepta=7 octa=8 nona=9 deca=10

Exercise: 7 6 5 4 3 2 1 3-ethyl-2,5-dimethyl-4-propylheptane

2,3,5-Trimethylhexane Not 2,4,5- 3-Ethyl-6-methyloctane

Notice: ① Prefixes: n-, sec-, tert- and iso, neo, are common names; ② When alphabetizing the substitutents, some prefixes (di, tri, tetra and n-, sec-, tert- and so on) should be ignored; but some prefixes ( iso, neo, cyclo) shouldn’t be ignored; ③ Be careful not to mix the two nomenclature systems. E.g. 2-methylisobutane is wrong.

Exercise: 3-ethyl-4-isopropyl-3-methyl-5-propyloctane

Properties 1. Physical properties of alkanes 2. Chemical properties of alkanes

1. Physical properties of alkanes Simple alkanes are colorless gases, clear liquids, or white solids, depending on their molecular weight. 36.1℃

(4) Density and solubility. d≈ 0.8g.mL-1 . Alkanes are hydrophobic. (1) C1～C4 gas, C5～C17 liquid, ≥ C18 solid. (2) molecular weight , bp. (3) branched , bp. bp(℃) 36.1 27.9 9.5 mp(℃) -129.8 -159.9 -16.8 (4) Density and solubility. d≈ 0.8g.mL-1 . Alkanes are hydrophobic.

2. Chemical properties of alkanes Stability: C－C and C－H bonds are quite strong, so alkanes are inert to many chemical reagents. Unaffected by bases, acids, reductants, or oxidants. Q4 Why are alkanes so inert? A reactive site in a molecule usually has one or more unshared pairs of electrons, a polar bond, an electron-deficient atom or an atom with an expandable octet. Alkanes have none of these.

Substitution reaction —— Halogenation of alkanes Methyl chloride Substitution reactions

CH3-Cl + Cl2 ——> CH2Cl2 + HCl Dichloromethane hv or heat CH2Cl2 + Cl2 ——> CHCl3 + HCl Trichloromethane hv or heat CHCl3 + Cl2 ——> CCl4 + HCl Tetrachloride hv or heat 400~450℃, CH4:Cl2=10:1，main product: CH3Cl. 400℃~ , CH4:Cl2=0.263:1，main product: CCl4.

Free radical chain reaction A. Reaction mechanism mechanism : How does the reaction take place? Free radical chain reaction

chain-initiating step chain-propagating step hv or △ chain-propagating step chain-terminating step

Other alkanes ~ methane The reactivity of halogenation of alkanes: F2 > Cl2 > Br2 > I2 Methane: violent not iodination Other alkanes ~ methane

B. Reactivity of hydrogen Q: For the above reaction, how many isomeric monochlorination products will produce? The ratio? 45% 55% Ratio of activity for 1ºH , 2ºH :

3°H > 2°H > 1°H > CH3-H Why？ CH3CH3 hv + Cl2 CH3Cl + CH3CH2Cl or Ratio: 1 400 63% 37% Ratio of activity for 1ºH and 3ºH: The order of reactivity for different hydrogen atoms is: Activity Order of H: 3°H > 2°H > 1°H > CH3-H Why？

DE. E CH3· RCH2· R2CH· R3C· CH3—H RCH2—H R2CH—H R3C—H ΔH=435kJ/mol

Q1: Which one is the most stable?（ ） A、CH3· B、(CH3)3C· C、CH3CH2CH2· D、(CH3)2CH· Q2: What hybridization does the methyl free radical (CH3·) adopt? How many electrons are there in its 2p orbital? 0, 1 or 2?

Geometry of Alkyl Radicals

C. Selectivity 2°/1°=（97/2）÷（3/6）= 97/1

SUMMARY Structure sp3 hybridization，tetrahedron Isomerism conformation: sawhorse formula & Newman projection Nomenclature IUPAC name: 3 steps 4. Properties Free radical substitution 5. Mechanism, Reactivity and Selectivity Reactivity: R3CH > R2CH2 > RCH3 ; F2> Cl2 > Br2 > I2