Alkanes and Cycloalkanes Lecture Notes
SYLLABUS THEME 2: ALKANES AND CYCLOALKANES
Course: Organic Chemistry II (OCA216X)
Textbook: Organic Chemistry by Janice Gorzynski Smith, 6th edition
Instructor: Ms Mangisa
SYLLABUS THEME 2: ALKANES AND CYCLOALKANES
STUDY UNIT 2.1: STRUCTURE AND REACTIONS
STUDY UNIT THEME 2.1.1: HYBRIDIZATION AND COVALENT BONDS OF CARBON
Specific Outcomes:
Explain the concept of hybridization in covalent bond formation.
Discuss the differences in reactivity of C-C bonds.
ORBITALS
Definition: An orbital is a region of space around the nucleus where the probability of finding an electron is high.
Types of Orbitals:
s orbitals:
Shape: Spherical.
p orbitals:
Shape: Dumbbell (or lobular), can orient along three mutually perpendicular directions.
d and f orbitals: Less common in organic chemistry.
ELECTRON SHELL
Arrangement: Orbitals are organized into layers, each with varying numbers and types of orbitals.
Shell Capacities:
1st shell (1s): Holds 2 electrons.
2nd shell (2s and 2p): Holds 8 electrons.
3rd shell (3s, 3p, 3d): Holds 18 electrons.
ELECTRON CONFIGURATIONS
Ground-State Configuration: The list of orbitals occupied by an atom's electrons.
Filling Order: Follow these rules for electron configurations:
Rule 1: Fill orbitals of the lowest energy first:
Order: .
Rule 2: An orbital can hold two electrons of opposite spins.
Rule 3: If multiple degenerate orbitals are available, single electrons occupy each before pairing occurs (Hund’s Rule).
DEVELOPMENT OF CHEMICAL BONDING THEORY
Contributors: Jacobus van’t Hoff and Joseph Le Bel proposed that the four bonds of carbon have a specific spatial orientation, forming a regular tetrahedron.
Bond Diagram Representation:
Solid line: Bonds in the plane of the page.
Wedge line: Bond coming out of the page toward the viewer.
Dash line: Bond receding behind the page from the viewer.
THE NATURE OF CHEMICAL BONDS
Stability: Atoms bond as a result of the formation of a more stable compound than the separate atoms.
Types of Bonds:
Ionic Bonds: Result from electron transfer.
Covalent Bonds: Result from sharing electrons.
Lewis Structures: Show valence electrons as dots (e.g., Hydrogen = 1 dot; Carbon = 4 dots,) indicating unpaired electrons available for bonding.
Noble Gas Configuration: The stable arrangement achieved by all atoms (octet rule: eight dots for main-group atoms, and two for hydrogen).
Kekulé Structures: Line-bond structures where lines indicate covalent bonds (two electrons).
FORMING COVALENT BONDS: VALENCE BOND THEORY
Valence Bond Theory: A covalent bond forms when two atoms approach closely and overlap singly occupied orbitals.
Example: The formation of H–H bonds from overlapping 1s orbitals.
sp3 HYBRID ORBITALS AND THE STRUCTURE OF METHANE
Formula: For methane (CH4), carbon has four valence electrons () that result in 4 covalent bonds.
Hybridization: Proposed by Linus Pauling, where one s and three p orbitals combine to create four equivalent sp3 hybrid orbitals with tetrahedral orientation.
Naming: 'sp3' denotes the combination ratio of orbitals, not the electron occupancy.
Conceptual Understanding: Hybridization explains the formation of four equivalent tetrahedral bonds around carbon.
THE STRUCTURE OF METHANE
Bond Details:
Each C-H bond has:
Strength: 436 kJ/mol (104 kcal/mol)
Length: 109 pm (1.09 Å)
Bond Angle: Each H–C–H angle: 109.5° (tetrahedral angle).
sp3 HYBRID ORBITALS AND THE STRUCTURE OF ETHANE
Bond Formation: Ethane (C2H6) involves the bonding of two carbons via sp3 hybrid orbital overlap, along with C-H bonds formed from overlap with hydrogen's 1s orbitals.
Details:
Each C-H bond is similar in character to methane, though slightly weaker.
Bond Strength: 421 kJ/mol (101 kcal/mol) for C-H, and for C-C, 377 kJ/mol (90 kcal/mol).
Bond Length: C-C = 154 pm.
Bond Angles: 109.5° (tetrahedral).
OTHER KINDS OF HYBRID ORBITALS: SP2 AND SP
Hybridization Types:
sp3: 2s hybridizes with three 2p orbitals to form four sp3 orbitals.
sp2: 2s hybridizes with two of three 2p orbitals, resulting in three sp2 orbitals with one unhybridized 2p orbital.
sp: 2s hybridizes with one 2p orbital, producing two sp orbitals, with two unhybridized 2p orbitals remaining.
sp2 HYBRID ORBITALS AND THE STRUCTURE OF ETHYLENE
Geometry:
sp2 orbitals lie in a plane with bond angles of 120°.
The unhybridized p orbital is perpendicular to the sp2 plane.
Bond Formation:
Two sp2 carbons form a sigma bond through head-on overlap with a p orbital contributing to a pi bond (sideways overlap).
sp2 HYBRID ORBITALS AND THE STRUCTURE OF ETHYLENE
Bond Characteristics:
C=C Double Bond: Stronger and shorter than single C-C bonds, with a bond strength of 728 kJ/mol (174 kcal/mol) and a length of 134 pm.
sp HYBRID ORBITALS AND THE STRUCTURE OF ACETYLENE
Hybridization: Carbon's 2s orbital hybridizes with a single p orbital to form two sp orbitals, while two remaining p orbitals are unhybridized.
Arrangement: sp orbitals are linear with a bond angle of 180°.
Triple Bond Formation: C≡C triple bond consists of one sigma bond and two pi bonds from p orbital overlaps.
TRIPLE BOND DETAILS
Bond Length: 120 pm.
Strength: 965 kJ/mol (231 kcal/mol).
Bond Angle: 180°.
Implication: This results in a stable, linear molecule formation for acetylene.
EXERCISES
Exercise to Determine Hybridization State:
H2C = C = CH – CH3
Answer: C1-sp2, C2-sp, C3-sp2, C4-sp3
(a) C1-sp3, C2-sp3, C3-sp2, O4-sp3;
(b) Geometry:
C1-tetrahedral
C2-tetrahedral
C3-trigonal planar
O4-tetrahedral
STUDY UNIT THEME 2.1.2: ISOMERISM AND NOMENCLATURE
Specific Outcomes:
Classify carbons according to substitution patterns: primary, secondary, tertiary, or quaternary.
Define and give examples illustrating homologous series, isomers, constitutional isomers, rotational isomers, cis and trans isomers.
Provide IUPAC names for alkanes and cycloalkanes.
ALKANES
ALKYL GROUPS: ISOMERS
Description: Alkanes are generated by the linkage of three or more carbon atoms through C-C single bonds.
Characteristics:
Saturated hydrocarbons (contain only C-C and C-H single bonds).
General Formula: (where n = number of carbon atoms).
ALKANES EXAMPLES
Molecules:
CH4 = Methane
C2H6 = Ethane
C3H8 = Propane
ISOMERISM IN ALKANES
Isomer Types:
Alkanes with more than three carbons can have multiple structures.
C4 (Butane):
Isomer: Butane and Isobutane
C5 (Pentane):
Isomers: Pentane, 2-Methylbutane (Isopentane), and 2,2-Dimethylpropane (Neopentane)
ISOMERS DEMONSTRATIONS
Representation of Alkanes:
Condensed Structures represent molecules without showing bonds:
Ex. Propane: ;
Ex. 2,2-Dimethylpropane:
DEFINTIONS OF ISOMERS
Isomers: Molecules with the same formula but different structures (from Greek "isos" + "meros" meaning same parts).
Constitutional Isomers: Molecules like butane and isobutane that differ in the connection of atoms.
ALKYL GROUPS
Definition: When a hydrogen is removed from an alkane, the resulting structure is an alkyl group.
Naming: Alkyl groups are named by replacing the -ane ending with -yl.
Types of Alkyl Groups:
Primary (end of chain), Secondary (middle of chain), Tertiary (central carbon with three carbons attached).
Symbol R: Represents a generalized alkyl group (e.g., methyl, ethyl).
NOMENCLATURE OF BRANCHED-CHAIN ALKANES
Components of IUPAC Naming:
Prefix: Specifies location and identity of substituents.
Parent: Indicates main molecule and counts the carbon atoms.
Locant: Numbering for the primary functional group.
Suffix: Indicates primary functional group.
NAMING RULES AND PROCEDURES
Naming Structure:
Longest continuous carbon chain determines the parent chain.
Number carbon atoms in sequence, with lowest numbers for substituents.
Multiple Substituents: If two substituents exist, the first one alphabetically assigns the lowest number.
CYCLOALKANES
Definition: Cycloalkanes are alkanes forming a ring structure (also referred to as alicyclic compounds).
General Formula: , where (CH2)n represents the number of carbon units in the ring.
NAMING STEPS FOR CYCLOALKANES
Find Parent: Count carbons in the ring and the longest substituent chain.
If ring number exceeds or equals substituent count, name it as alkyl-substituted cycloalkane.
Number Substituents: Start at the attachment point, numbering around the ring, giving the lowest numbers for substituents.
CIS-TRANS ISOMERISM IN CYCLOALKANES
Clear Similarities and Differences:
Both cycloalkanes and acyclic alkanes are non-polar and chemically inert to many reagents.
Acyclic alkanes permit free rotation around C-C bonds, while cycloalkanes have less rotational freedom due to their rigid structure.
STEREOCHEMISTRY
Definition: The study of three-dimensional arrangements in chemical structures.
Stereoisomers: Isomers with the same atom connectivity but different spatial arrangements.
STUDY UNIT 2.1.3: CONFORMATION AND STEREOISOMERS
Specific Outcomes:
Draw conformational and rotational isomers.
Draw Newman projections and explain stabilities of cyclohexane conformations.
Distinguish axial and equatorial substituents in cyclohexanes.
CONFORMATION OF ETHANE
Rotation: Ethane has free rotation around its C-C single bonds due to head-on overlap during bond formation.
Definitions:
Conformation: Various arrangements from bond rotation.
Conformational Isomers: Molecules with different arrangements but capable of interconversion.
CONFORMATIONS OF ETHANE
Representation methods:
Sawhorse representation: Indicates spatial relationships from an oblique angle, showing all C-H bonds.
Newman projection: Representing C-C bonds end-on, depicting bonds as lines radiating from a circle representing the front carbon.
CONFORMATION OF BUTANE
Eclipsed and Staggered Conformations: Different energy levels—staggered (most stable, lowest energy) and eclipsed (least stable, higher energy).
Conformational Energy Analysis: Demonstrates energy variance due to interactions among hydrogen atoms during arrangements.
CONFORMATION OF CYCLOALKANES
Cyclopropane's Structure: Flat, triangular with C-C-C bond angles of 60°, allowing for angle strain and increased reactivity.
Characteristics of Cyclobutane and Cyclopentane: Slightly puckered arrangements reduce angle strain and create unique bond characteristics.
Cyclohexane: Notable for its stable chair conformation, characterized by axial and equatorial positions, alternates between these arrangements during thermal transitions (ring-flip).
STRAIN-FREE CHAIRS
Chair Conformation: Free from angle and torsional strains, with alternating planar arrangements resulting in a stable configuration.
Substituent Positioning: Defined by axial positions (perpendicular to the ring) and equatorial positions (around the ring's equator).
SYLLABUS THEME 2: ALKANES AND CYCLOALKANES
The course focused on Organic Chemistry II (OCA216X) is guided by the textbook "Organic Chemistry" by Janice Gorzynski Smith, 6th edition, under the instruction of Ms. Mangisa.
STUDY UNIT 2.1: STRUCTURE AND REACTIONS
In Study Unit 2.1, the emphasis is on hybridization and covalent bonds of carbon. The specific outcomes include explaining the concept of hybridization in covalent bond formation and discussing the differences in reactivity of C-C bonds.
ORBITALS
An orbital is defined as a region of space around the nucleus where the probability of finding an electron is high. The main types of orbitals include s orbitals, which are spherical in shape, p orbitals, which are shaped like dumbbells and orient along three mutually perpendicular directions, and d and f orbitals, which are less common in organic chemistry.
ELECTRON SHELL
Orbitals are organized into layers, known as electron shells, each varying in the number and types of orbitals. The capacities for these shells are as follows: the 1st shell (1s) holds 2 electrons, the 2nd shell (2s and 2p) holds 8 electrons, and the 3rd shell (3s, 3p, and 3d) holds 18 electrons.
ELECTRON CONFIGURATIONS
The ground-state configuration refers to the list of orbitals occupied by an atom's electrons. The filling order follows specific rules: Rule 1 states to fill orbitals of the lowest energy first in the order of . Rule 2 indicates that an orbital can hold two electrons of opposite spins, and Rule 3, also known as Hund’s Rule, states that if multiple degenerate orbitals are available, single electrons occupy each before pairing occurs.
DEVELOPMENT OF CHEMICAL BONDING THEORY
Jacobus van’t Hoff and Joseph Le Bel contributed to the chemical bonding theory by proposing that the four bonds of carbon have a specific spatial orientation, forming a regular tetrahedron. Bond diagrams represent these bonds with solid lines indicating bonds in the plane of the page, wedge lines for bonds coming out of the page, and dashed lines for bonds receding behind the page.
THE NATURE OF CHEMICAL BONDS
Atoms bond to form a more stable compound than when they are separate. The two main types of bonds are ionic bonds, which result from electron transfer, and covalent bonds, which result from sharing electrons. Lewis structures display valence electrons as dots (e.g., Hydrogen = 1 dot; Carbon = 4 dots) that indicate unpaired electrons available for bonding, adhering to the noble gas configuration principle and the octet rule.
FORMING COVALENT BONDS: VALENCE BOND THEORY
According to Valence Bond Theory, a covalent bond forms when two atoms approach closely and overlap singly occupied orbitals, exemplified by the formation of H–H bonds from overlapping 1s orbitals.
sp3 HYBRID ORBITALS AND THE STRUCTURE OF METHANE
For methane (CH4), with carbon having four valence electrons (), it results in four covalent bonds. Proposed by Linus Pauling, hybridization involves the combination of one s and three p orbitals to create four equivalent sp3 hybrid orbitals, oriented tetrahedrally. The term 'sp3' denotes the combination ratio of orbitals. This concept aids in understanding the formation of the equivalent tetrahedral bonds around carbon.
In detailing the structure of methane, each C-H bond has a strength of 436 kJ/mol (104 kcal/mol), a length of 109 pm (1.09 Å), and a bond angle of 109.5° (tetrahedral angle).
sp3 HYBRID ORBITALS AND THE STRUCTURE OF ETHANE
The bond formation in ethane (C2H6) occurs through sp3 hybrid orbital overlap between two carbon atoms, with C-H bonds formed from overlap with hydrogen's 1s orbitals. Each C-H bond is similar to methane but slightly weaker, having a bond strength of 421 kJ/mol (101 kcal/mol) for C-H and 377 kJ/mol (90 kcal/mol) for C-C. The bond length for C-C is 154 pm, with bond angles at 109.5°.
OTHER KINDS OF HYBRID ORBITALS: SP2 AND SP
There are various hybridization types: sp3, which involves the hybridization of 2s with three 2p orbitals to form four sp3 orbitals; sp2, where 2s hybridizes with two 2p orbitals resulting in three sp2 orbitals and one unhybridized 2p orbital; and sp, where 2s hybridizes with one 2p orbital to produce two sp orbitals, leaving two unhybridized 2p orbitals.
sp2 HYBRID ORBITALS AND THE STRUCTURE OF ETHYLENE
The sp2 orbitals lie in a plane with bond angles of 120°, while the unhybridized p orbital is perpendicular to this plane. Two sp2 carbons form a sigma bond through head-on overlap, with the p orbitals contributing to a pi bond via sideways overlap. The characteristics of the C=C double bond in ethylene make it stronger and shorter than single C-C bonds, showing a bond strength of 728 kJ/mol (174 kcal/mol) and a length of 134 pm.
sp HYBRID ORBITALS AND THE STRUCTURE OF ACETYLENE
Carbon's 2s orbital hybridizes with a single p orbital to create two sp orbitals, while two remaining p orbitals remain unhybridized. The arrangement of sp orbitals is linear with a bond angle of 180°. The formation of a C≡C triple bond comprises one sigma bond and two pi bonds that arise from p orbital overlaps, characterized by a bond length of 120 pm and a strength of 965 kJ/mol (231 kcal/mol).
EXERCISES
An exercise to determine hybridization states includes a sample structure H2C = C = CH – CH3, with the answer being C1-sp2, C2-sp, C3-sp2, C4-sp3. A second example is C1-sp3, C2-sp3, C3-sp2, O4-sp3, with geometrical designations of tetrahedral for C1 and C2, trigonal planar for C3, and tetrahedral for O4.
STUDY UNIT THEME 2.1.2: ISOMERISM AND NOMENCLATURE
The specific outcomes for this unit involve classifying carbons by substitution patterns such as primary, secondary, tertiary, or quaternary. It covers defining and providing examples for homologous series, isomers, constitutional isomers, rotational isomers, as well as cis and trans isomers. Furthermore, it aims to teach IUPAC naming for alkanes and cycloalkanes.
ALKANES
Alkanes are generated by linking three or more carbon atoms through C-C single bonds, categorized as saturated hydrocarbons that contain only C-C and C-H single bonds. The general formula for alkanes is given as , where n is the number of carbon atoms. Examples of alkanes include CH4 for methane, C2H6 for ethane, and C3H8 for propane.
ISOMERISM IN ALKANES
Alkanes with more than three carbons can exhibit multiple structures, as illustrated with C4 having isomers such as butane and isobutane, while C5 can have pentane, 2-Methylbutane (Isopentane), and 2,2-Dimethylpropane (Neopentane) as its respective isomers.
ISOMERS DEMONSTRATIONS
In representing alkanes, condensed structures can show molecules without displaying bonds, for instance, representing propane as and 2,2-Dimethylpropane as .
DEFINITIONS OF ISOMERS
Isomers are defined as molecules that share the same formula but differ in structure, derived from Greek "isos" and "meros" meaning same parts. Constitutional isomers are exemplified by butane and isobutane, differing in how the atoms are connected.
ALKYL GROUPS
When a hydrogen is removed from an alkane, the structure that remains is termed an alkyl group, with naming convention being alkyl groups derived from alkanes by replacing the ending -ane with -yl. The classification includes primary (located at the end of the chain), secondary (middle of the chain), and tertiary (central carbon with three carbons attached). The symbol R is commonly used to represent a generalized alkyl group (e.g., methyl, ethyl).
NOMENCLATURE OF BRANCHED-CHAIN ALKANES
IUPAC naming includes components such as prefixes to specify the location and identity of substituents, a parent name indicating the main molecule and the carbon count, locants for functional group numbering, and suffixes that specify primary functional groups.
NAMING RULES AND PROCEDURES
To establish a naming structure, the longest continuous carbon chain identified determines the parent chain, with carbon atoms numbered sequentially to afford the lowest numbers to substituents. In cases with multiple substituents, the first one alphabetically is assigned the lowest number.
CYCLOALKANES
Cycloalkanes are characterized as alkanes forming ring structures, also termed alicyclic compounds, with a general formula of , representing the number of carbon units in the ring. The naming of cycloalkanes follows a systematic approach, first counting carbons in the ring and the longest substituent chain; a ring number that meets or exceeds the number of substituents leads to naming as an alkyl-substituted cycloalkane.
CIS-TRANS ISOMERISM IN CYCLOALKANES
Cyclic alkanes, like acyclic alkanes, are non-polar and chemically inert towards many reagents. However, acyclic alkanes allow free rotation around C-C bonds, while cycloalkanes possess limited rotational freedom owing to their rigid structure.
STEREOCHEMISTRY
Stereochemistry focuses on the three-dimensional arrangements in chemical structures, with stereoisomers having identical atom connectivity yet differing in spatial arrangement.
STUDY UNIT 2.1.3: CONFORMATION AND STEREOISOMERS
The specific outcomes include drawing conformational and rotational isomers, illustrating Newman projections while explaining the stability of cyclohexane conformations and distinguishing between axial and equatorial substituents in cyclohexanes.
CONFORMATION OF ETHANE
Ethane allows for free rotation around its C-C single bonds due to head-on overlap resulting during bond formation. Various arrangements from bond rotation are termed conformations, leading to configurational isomers which differ but can interconvert.
CONFORMATIONS OF ETHANE
Representation methods include sawhorse representation, which indicates spatial relationships from an angle, presenting all C-H bonds, and Newman projection, where C-C bonds are depicted end-on, with lines radiating from a circle representing the front carbon.
CONFORMATION OF BUTANE
Butane shows eclipsed and staggered conformations at varying energy levels, with staggered being most stable at a lower energy, whereas eclipsed is least stable at higher energy. Conformational energy analysis reflects energy differences attributed to hydrogen interactions during arrangements.
CONFORMATION OF CYCLOALKANES
Cyclopropane features a flat triangular structure with C-C-C bond angles of 60°, resulting in angle strain and increased reactivity. Cyclobutane and cyclopentane present slightly puckered arrangements that reduce angle strain and engender distinct bond characteristics, with cyclohexane recognized for its stable chair conformation featuring axial and equatorial positions that alternate through thermal transitions known as ring-flip.
STRAIN-FREE CHAIRS
Chair conformation is devoid of angle and torsional strains and shows stable configurations through alternating planar arrangements. The positioning of substituents distinguishes between axial (perpendicular to the ring) and equatorial (situated around the ring's equator).
This course covers Organic Chemistry II and is guided by a textbook by Janice Gorzynski Smith. Instruction is provided by Ms. Mangisa.
In the first unit, the focus is on understanding carbon atoms, how they bond together, and their different arrangements. Carbon has various types of atomic orbitals where electrons are found. The main types include:
s orbitals: round shape
p orbitals: shaped like dumbbells and oriented in three directions.
Atoms are organized into electron shells, with the first shell holding 2 electrons, the second shell holding up to 8, and the third holding 18. To describe how electrons occupy these shells, specific rules apply. The main rule is that lower energy orbitals fill first.
Chemists, like Jacobus van't Hoff, studied how carbon bonds form a shape called a tetrahedron. This allows us to understand how stable compounds are made when atoms join together. There are two main types of chemical bonds: ionic (when electrons are transferred) and covalent (when electrons are shared).
In covalent bonding, two atoms come together to form a bond by overlapping their electrons. For example, in methane (CH4), carbon shares its four electrons to create four bonds. In ethane (C2H6), two carbon atoms are connected by these bonds as well.
When carbons bond together to create structures, they can form different shapes or structures called isomers. For example, butane can exist in two forms: butane and isobutane.
When we take away a hydrogen from an alkane, we form something called an alkyl group. The naming of these groups follows specific rules based on the structure they create.
Cycloalkanes are alkanes that form ring structures. For naming, we count the carbons and determine the longest chain to define the molecule's name. There are also some special cases in cycloalkanes called cis-trans isomers based on how the substituents are positioned.