**Spin-Spin Coupling in H-NMR Spectrum**

Spin-spin coupling is a phenomenon observed in H-NMR spectra where the signals of neighboring protons influence each other, leading to the splitting of resonance peaks. The pattern of spin-spin coupling can provide valuable information about the structural arrangement of protons in a molecule.

In the given compound, Br2CHCH2Br, there are two types of protons: the protons attached to the central carbon (CH) and the protons attached to the terminal carbons (CH2). Let's analyze the spin-spin coupling pattern for each type of proton and explain why certain protons appear in the downfield region.

**Spin-Spin Coupling in CH Protons:**

The CH protons are directly bonded to the central carbon atom, which is also bonded to two bromine atoms. Hence, each CH proton will experience spin-spin coupling with the equivalent neighboring protons.

- Each CH proton has two equivalent neighboring protons (from the CH2 group) that are three bonds away. This coupling is known as a **3J coupling**.

- The spin-spin coupling between the CH protons and the CH2 protons will result in a **doublet pattern** in the H-NMR spectrum. The CH proton signal will be split into two peaks, with a relative intensity ratio of 1:1. The splitting is caused by the **n+1 rule**, where n is the number of equivalent neighboring protons.

- Since each CH proton has two equivalent neighboring protons, the resulting doublet peaks will appear as a **doublet of doublets** in the H-NMR spectrum.

**Spin-Spin Coupling in CH2 Protons:**

The CH2 protons are directly bonded to the terminal carbon atoms, which are also bonded to bromine atoms. Each CH2 proton will experience spin-spin coupling with the equivalent neighboring protons.

- Each CH2 proton has two equivalent neighboring protons (from the CH group) that are three bonds away. This coupling is also known as a **3J coupling**.

- The spin-spin coupling between the CH2 protons and the CH protons will result in a **quartet pattern** in the H-NMR spectrum. The CH2 proton signal will be split into four peaks, with a relative intensity ratio of 1:3:3:1. The splitting is again caused by the **n+1 rule**, where n is the number of equivalent neighboring protons.

- Since each CH2 proton has two equivalent neighboring protons, the resulting quartet peaks will appear as a **doublet of quartets** in the H-NMR spectrum.

**Protons in the Downfield Region:**

In NMR spectroscopy, the chemical shift is a measure of the electron density around the proton and is influenced by the local environment of the proton. Electronegative atoms, such as halogens, can deshield nearby protons, causing them to appear at higher chemical shifts (downfield) in the NMR spectrum.

In the given compound, the CH2 protons are closer to the bromine atoms compared to the CH protons. The bromine atoms, being highly electronegative, deshield the CH2 protons more effectively than the CH protons. Therefore, the CH2 protons will appear in the downfield region of the H-NMR spectrum.

In conclusion, the spin-spin coupling pattern