Lecture 2. The framework to build materials and understand properties

Transcription

1 Lecture 2 The framework to build materials and understand properties 1

2 Trees are made into a solid materials/structures in an environment that consists of small molecules: C 2, N 2, H 2 0, CH 4 C 2.58Ǻ? 40 m 2

3 Why do molecules reside in a liquid or solid state at room temperature? r what are the forces that hold atoms and molecules together? 3

4 Methods of developing solid materials Gas Gas Liquid Liquid Solid Solid Temperature or 1/Pressure Molecule size 4

5 A material s property is dictated by Molecule size Chemical composition Packing These parameters influence the type of interactions amongst atoms in a molecule and interactions between molecules 5

6 Some Basic Information on Condensed Matter Atomic structure dictates properties Nucleus of protons and neutrons, with electron shell (atoms are overall neutral) Atoms and the Pauli Exclusion principle: No two electrons in an atom can have the same set of four quantum numbers. Electrostatic interactions cause attraction between particles 6

7 Atoms associate due to Primary and secondary bonds Ionic and covalent Dipole bonds Hydrogen bonds London dispersion forces Metallic bonds C. Frazier 7

8 Covalent bonds form when two atoms of same or similar electronegativities react and share electrons Nonpolar covalent bond is when the distribution of the electron density is symmetrical bonding is symmetrical between two nuclei = Polar covalent bond is when the distribution of the electron density is asymmetrical and there is an electronegativity difference > 0.4 More background info 8

9 Electronegativity: measure of the ability to attract electrons Ionic compounds (salts) will form from the reaction of atoms of greatly differing electronegativities (interact by gaining or losing electrons) More background info m/watch?v=jjzgw5nazk

10 DIPLES make polar molecules Spatially distinct poles on single material (remember asymmetric sharing of electrons) Poles are electronic or magnetic For intermolecular forces we discuss electronic poles Partial charges, δ + and δ -, that are either temporary or permanent on the molecule Electrons are not shared equally 10

11 All atoms and molecules can be polarizable (related to size) and bond type The induced dipole moment is based on both a chemical characteristic and the applied field Electron cloud Nucleus 11

12 Image is from MLCAD GmbH 12

13 Non-polar molecule Ion with field Non-polar molecule with instantaneous dipole 13

14 Intermolecular forces limit free motion of molecules in space Hydrocarbon Methane -161 Ethane -88 Propane -42 Butane 0 Boiling point ( C) Larger molecules undergo increased intermolecular interactions (dispersion forces increase with molar mass size) Temporary dipoles

15 Added permanent dipoles increase interactions with H Methanol Boiling point ( C) 65 Ethanol 78 1-Propanol 98 1-Butanol 118 Plants make glucose via photosynthesis, a carbohydrate C(H2) that has 5/6 free hydroxyl groups With 2 H Ethylene glycol 196 Permanent dipoles

16 Hydrogen bonds A specific type of intermolecular bond Molecules must have H attached to strongly electronegative atoms such as (, N, and F*) -creates a permanent dipole and hydrogen is unique in size and electron content xygen containing compounds, if have accessible lone pair electrons, can be hydrogen bond acceptors More background info 16

17 Bond Strengths Covalent Bonds Ionic Bonds Hydrogen bonds Dipole-Dipole Dipole-induced dipole KJ/mole KJ/mole KJ/mole 4-20 KJ/mole <2 KJ/mole London (induced-induced) KJ/mole 1/r 6 17

18 Cell wall of plants have cellulose, lignin and a collection of hemicelluloses. No chemical bonds between cellulose chains and other components. Hydrogen bonds everywhere in wood! H H Cellulose H H H H H Xylan backbone H H H H H H H H 2 C H C C C C H H H Mannose H C H H H H 3 C H H 3 C CH 3 Schematic from Nanotructural assembly of cellulose, hemicellulose, and lignin in the middle layer of secondary wall of ginkgo tracheid. by N. Terashima and co-workers, in J wood Sci (2009) 55: Some representation of lignin

19 Lot s of intermolecular interactions in the cell wall of plants This schematic is from an article Micromechanical Understanding of the cellwall structure by Lennart Salmén in C.R. Biologies 327 (2004) Schematic from Nanotructural assembly of cellulose, hemicellulose, and lignin in the middle layer of secondary wall of ginkgo tracheid. by N. Terashima and co-workers, in J wood Sci (2009) 55:

20 Zhu et al Delignification removes components in the cell wall by breaking covalent bonds H H H H H H H Li et al Cellulose remains intact afterwards 20

21 Materials are influenced by bond type Thermal Energy (KT) at 25C (J) 4.1E-21 Bond type Bond energy J/bond (examples) bond energy/ KT covalent 3.321E dipole 2.491E disper 6.642E London Dispersion bonds are between non-polar molecules Dipole and hydrogen bonds are between polar molecules 21

22 So the ability of a molecule to interact will be dictated by chemical composition Functional groups can be used to help classify chemical composition Responsible for chemical reactivity and interactivity 22

23 Hydrocarbons Alkanes (aliphatics) alkyl groups, drop an H Alkenes Alkynes Aromatics Benzene, phenyl, and benzyl groups H 2 C CH 2 HC CH CH 3 CH4 Review octet rules

24 Alcohols Hydroxyl (-H) vs. alcohols (R-H) Hydroxyl derivative of alkane vs. alkyl derivative of water Relatively familiar alcohols Methanol, ethanol, propanol Phenol H 3 C H H 3 C H 2 C H H 24

25 Amines rganic derivatives of ammonia Draw ammonia Draw ethylamine H H N H 25

26 Ethers Ethers have an R--R or R--R Cyclic ethers xacyclopropane xirane/epoxide Ethylene oxide 26

27 Aldehyde vs. Ketone A carbonyl group is a carbon atom that has a double bond to oxygen Aldehyde has RCH and ketone has RCR Acetone is a simple ketone CH3-C-CH3 Formaldehyde is the simplest aldehyde -- HCH 27

28 Carbonyls: carboxylic acid Carbonyls that are bonded to an alcohol (RC 2 H) is a carboxylic acid Carbonyl and hydroxyl group together is known as carboxyl group (C 2 H or CH) 28

29 Carbonyl: Esters Esters are when carbonyl groups are bonded to an alkoxyl group (-R) General formula of (RCR ) Based on chain length of R common esters are Acetate, propanoate, butyrate 29

30 To summarize Subatomic particles give rise to interactions amongst atoms and molecules Covalent bonds between atoms can be classified as polar or nonpolar, depending what atoms are bonded together Secondary interaction strength is influenced by chemical composition Functional groups dictate the reactivity or interactivity of a molecule and used to classify specific arrangements 30